and Meckelian cartilages) are supplemented by the addition of certain
tooth-bearing membrane bones which form secondary jaws corresponding to the
functional jaws of the higher Craniates.[560] The chondrocranium and the
primary jaws are usually more or less completely ossified by cartilage
bones, and there is always a secondary cranium of dermal bones, of which
paired parietals and frontals above, and a median vomer and a parasphenoid
below, are amongst the most constant. The skull is hyostylic. An operculum
covering the gill-clefts and supported by a special opercular skeleton is a
constant feature. The vertebral column is often acentrous, and when centra
are present they are invariably arch-centra. There is a well-developed
secondary pectoral girdle, connected dorsally with the hinder part of the
skull. As a rule the pelvic girdle is absent altogether, and when present
it is rarely more than a rudiment or a vestige. The endoskeletal supports
of the paired fins are uniserial. The dermal fin-rays of the paired and
median fins are probably modified scales or lepidotrichia. In the median
fins the fin-rays are at first more numerous than their supporting radials,
but in the more specialised Teleostomes they ultimately equal them in
number. The body is usually invested by an exoskeleton of articulated
rhombic or imbricated cycloid scales. Claspers are unknown. In the
surviving members of the group there is usually an {476}air-bladder. The
gill-filaments project freely beyond the outer edges of the greatly reduced
interbranchial septa. The external opening of each nasal sac is usually
divided into two distinct apertures, and there is no oro-nasal groove
leading from the sac to the mouth. The brain has no proper cerebral
hemispheres, but retains an undivided prosencephalon with a non-nervous
roof. A cloaca is not developed, the rectum opening externally by an anus
in front of, and distinct from, the separate or united urino-genital
apertures. The ova are small and numerous, and the segmentation is either
holoblastic and unequal, or meroblastic. Besides a large number of fossil
forms the group includes the vast majority of living Fishes.
The Teleostomi include four "Orders," the CROSSOPTERYGII, the CHONDROSTEI,
the HOLOSTEI, and the TELEOSTEI. Of these the Crossopterygii occupy a
remarkably central position. Remotely connected with the Elasmobranchs on
the one hand, and more intimately related to the Holostei and Teleostei on
the other, they also probably represent the ancestral stock from which the
Stegocephalan Amphibia and the Dipneusti have had their origin. Of the
three remaining groups, often collectively spoken of as "Actinopterygii,"
the Chondrostei are the oldest and most primitive. Like the Crossopterygii,
they are not without evidence of a remote kinship with the Elasmobranchs,
but in a broad general sense they also represent the initial stages in a
sequence of structural modifications, of which the Teleostei, the dominant
Fishes of the present day, are the final outcome.
ORDER I. CROSSOPTERYGII.
Pectoral fins obtusely lobate and probably uniserial, or acutely lobate and
probably biserial. Pelvic fins abdominal in position, uniserial,
non-lobate, or obtusely lobate. Scales rhombic or cycloid, and, like the
dermal cranial bones, they are generally invested by a layer of enamel-like
ganoin. Tail heterocercal, or apparently diphycercal or gephyrocercal.
Vertebral column acentrous, or with ring-like centra, or even with complete
bony amphicoelous centra. Lower jaw with dentigerous splenials. As a rule,
the opercular series includes an operculum and a suboperculum.
Branchiostegal rays absent, their place being taken by a remarkable
armature of jugular plates (Fig. 274). Secondary pectoral girdle
{477}complete, including a pair of infra-clavicles. With rare exceptions
the fin-rays of the median fins retain their numerical preponderance over
the supporting radials. The group is divisible into two "sub-orders," the
OSTEOLEPIDA and the CLADISTIA.[561]
SUB-ORDER 1. OSTEOLEPIDA.
The obtusely or acutely lobate pectoral fins articulate with the pectoral
girdle by a single basal endoskeletal element. Nostrils on the ventral
surface of the snout. Two dorsal fins and an anal fin. Dermal bones of the
ethmoid region often fused with one another and with the premaxillae in
front and the frontals behind to form a continuous rostral shield.
Infra-dentary bones may be present. A series of lateral jugular plates
often present in addition to the pair of principal plates. The Osteolepida
first make their appearance in the Old Red Sandstone and Devonian
formations, where they become abundant. They are also well represented in
the Carboniferous, but only one family survived to the Mesozoic period,
finally becoming extinct in the Upper Cretaceous. The following are the
more important families:—
[Illustration: FIG. 273.—Restoration of _Osteolepis macrolepidota_. Old Red
Sandstone. (From Traquair.)]
FAM. 1. OSTEOLEPIDAE.—Scales rhombic and thickly enamelled. Pectoral and
pelvic fins obtusely lobate. Tail heterocercal. Teeth simple, not
complicated by surface infoldings except quite at the base.
Genera:—_Osteolepis_ (Fig. 273), _Thursius_, _Diplopterus_ (Middle Old Red
Sandstone, Scotland), _Glyptopomus_ (Upper Old Red Sandstone, Scotland),
{478}_Megalichthys_ (Carboniferous and Lower Permian of Europe and North
America).
[Illustration: FIG. 274.—Skull of a Rhizodont (_Rhizodopsis sauroides_),
Lower Carboniferous. A, lateral view; B, the dorsal surface; and C, ventral
view. _an_, Angular; _d_, dentary; _f_, frontal; _i.d_, infra-dentary; _j_,
principal jugular plates; _l.j_, lateral jugulars; _m_, mandible; _m.j_,
median jugular; _mx_, maxilla; _o_, orbit; _op_, operculum; _p_, parietal;
_p.f_, post-frontal; _p.mx_, premaxilla; _p.op_, preoperculum; _so_,
suborbital; _s.op_, suboperculum; _sq_, squamosal; _st_, supra-temporal;
_x_, _x′_, cheek plates. (After Traquair.)]
FAM. 2. RHIZODONTIDAE.—Scales cycloid and overlapping. Paired fins obtusely
lobate. Tail heterocercal, sometimes apparently gephyrocercal. Teeth with
the external enamelled layer of dentine infolded towards the axis in the
form of radially arranged folds. In some genera ring-like vertebral centra
have been recognised and also a preoperculum. Genera:—_Rhizodus_, Lower
Carboniferous of Scotland and Northumberland; _Tristichopterus_[562] (Fig.
275), Old Red Sandstone of Scotland; _Eusthenopteron_[563] (Fig. 276),
Upper Devonian of Scaumenac {479}Bay, Canada; _Gyroptychius_, Old Red
Sandstone, Scotland; _Rhizodopsis_[564] (Fig. 274), Carboniferous of
England, Scotland, Silesia, and North America; _Strepsodus_, Carboniferous
of Great Britain, Ireland, and North America.
[Illustration: FIG. 275.—Restoration of _Tristichopterus alatus_. Old Red
Sandstone. _cl_, Clavicle. Remaining reference letters as in Fig. 274.
(After Traquair.)]
[Illustration: FIG. 276.—Restoration of _Eusthenopteron foordi_. Upper
Devonian of Scaumenac Bay, Canada. The scales have been omitted in the
hinder part of the body to show the vertebral column and the radials of the
median fins. _cl_, Clavicle; _i.cl_, infra-clavicle; _s.cl_,
supra-clavicle; for other reference letters see Fig. 274. × about ¼. (After
Whiteaves.)]
[Illustration: FIG. 277.—Restoration of _Holoptychius flemingi_. × ⅛. (From
Traquair.)]
FAM. 3. HOLOPTYCHIDAE (Dendrodontidae).—Scales cycloid. Pectoral fins
acutely lobate; pelvic fins short and somewhat obtusely lobate. Tail
heterocercal. Teeth similar to those of the Rhizodontidae but more
specialised, the enamelled dentine infoldings being much more complicated,
presenting a radiating {480}arborescent appearance in transverse sections.
Vertebral column acentrous. Genera:—_Holoptychius_[565] (Fig. 277), Old Red
Sandstone of Scotland; Devonian of Belgium, Russia, North America, and East
Greenland. _Glyptolepis_ has a similar range.
[Illustration: FIG. 278.—Restoration of _Undina gulo_. Lower Lias of
Dorset. Scales and supra-clavicle omitted. The ossified air-bladder is
shown beneath the anterior part of the vertebral column. The facial bones
in front of the orbit are unknown, and the cheek-plates are supposed to be
arranged as in other Coelacanths. × about ⅐. (From Smith Woodward.)]
FAM. 4. COELACANTHIDAE.[566]—Scales cycloid. Paired fins obtusely lobate.
Tail symmetrical but apparently gephyrocercal, usually with a protruding
axial vestige of the disappearing terminal part of the tail and of the
proper caudal fin. Radialia of the functional caudal lobes agree in number
with the contiguous neural and haemal arches and dermal fin-rays, the
diagnostic feature of Smith Woodward's Actinistia. Proximal radials of the
dorsal and anal fins fused into a single, internally-forked basipterygium
in each fin. Teeth simple. Vertebral column acentrous. The skull presents
several interesting features. The hyomandibular and the palato-quadrate
bar, for example, are fused on each side into a continuous triangular bone,
articulating with the cranium above and with the lower jaw below. The
opercular skeleton is reduced to an operculum and two jugular plates. A
very singular feature in these Fishes is the ossification of the walls of
the air-bladder (Fig. 278), a structural modification which has no parallel
in Fishes, except in certain Teleosts (Siluridae and Cyprinidae)[567]
{481}in which the organ becomes encapsuled by bone owing to the partial
ossification of its walls.
From their first appearance in the Lower Carboniferous the Coelacanthidae
range, practically unchanged, through the intervening formations to the
Upper Cretaceous. _Coelacanthus_ itself occurs in the Carboniferous and
Permian of England, Scotland, and Germany, and in the Carboniferous of
North America. _Undina_[568] (Fig. 278) is a Jurassic genus. _Diplurus_ is
found in the Trias of North America, and _Macropoma_ is a well-known form
from the Middle and Upper Cretaceous beds of England, and other parts of
Europe.
SUB-ORDER 2. CLADISTIA.
Pectoral fins uniserial and abbreviate, with three basal endoskeletal
elements. Nostrils on the upper surface of the snout. Entire skeleton well
ossified. Notochord replaced by bony, amphicoelous vertebral centra. Bones
of the ethmoid region not fused to form a rostral shield. Infra-dentary
bones absent. Jugular plates reduced to a single pair of large plates. As
this group includes the only Crossopterygii which have survived to the
present day, it is noteworthy that they retain certain primitive features
indicative of their remote origin. The spiracles are persistent; the
intestine has a spiral valve; and the conus arteriosus is furnished with
several rows of valves. Amongst other characters of contrary significance,
the air-bladder is double; its oesophageal aperture is ventral; and its
afferent arteries are pulmonary arteries derived from a posterior aortic
arch.
FAM. 5. POLYPTERIDAE.[569]—Pectoral fins obtusely lobate. Pelvic fins
non-lobate. Scales rhombic and thickly enamelled. Dorsal fin in the form of
a series of isolated finlets, each consisting of a stout spine-like[570]
fulcral scale supporting a single soft ray, or a fringe of several rays,
along its hinder margin. Tail symmetrical, apparently gephyrocercal. Teeth
simple. Nostrils tubular.
The only representatives of the sub-order and the sole {482}surviving
family of Crossopterygii, the Polypteridae, are restricted to the Nile and
to the river basins of tropical Africa which drain into the Atlantic (Fig.
280). Only two genera are known, _Polypterus_ and _Calamichthys_, neither
of which has yet been discovered in any geological deposits, ancient or
recent.
In _Polypterus_ each of the spines of the dorsal fin supports several soft
rays. Pelvic fins and a suboperculum are present. Ten species are known, of
which six pertain to the Congo and its tributaries.[571] _P. bichir_ is
said to attain a length of four feet.
Until recently little was known of the habits of _Polypterus_, but the
observations of Budgett[572] on the widely distributed _P. senegalus_ and
those of Harrington[573] on _P. bichir_, have brought to light many
interesting facts about these most interesting Fishes.
[Illustration: FIG. 279.—_Polypterus senegalus_. From a specimen in the
Cambridge University Museum. The arrow points to the position of the left
spiracle. × ⅓.]
_P. bichir_ haunts the deeper holes and depressions of the muddy bed of the
Nile, although it is "not essentially a bottom-liver or a mud-fish." It is
most active at night when in search of food, and then it may readily be
taken by trawl lines. The lobate pectoral fins are used for progression,
but their primary function is to act as balancers, and they exhibit the
characteristic trembling movements so often seen in the balancing fins of
Teleosts. _Polypterus_ does not readily live out of water, rarely longer
than three to four hours, and then only when covered with damp grass or
weeds. _P. bichir_ is said to feed on small Teleosts, which it swallows
whole, and to these there may be added in other species, Batrachians and
Crustaceans. The observations of Budgett show that in captivity
_Polypterus_ often remains motionless for a long time at the bottom of the
water, the anterior part of the body resting upon the tips of the
{483}pectoral fins. According to the same observer, the air-bladder is an
accessory respiratory organ, supplementary to the gills, rather than a
hydrostatic organ.
[Illustration: FIG. 280.—Map showing the distribution of the Polypteridae.]
In _P. bichir_ the eggs ripen from June to September, inclusive, and, as in
most other Nile Fishes, the breeding season is during or just after the
period of inundation. _P. senegalus_ and _P. lapradei_ spawn during the
rainy season in the months of July, August, and September, but nothing is
certainly known as to the place or mode of deposition of the eggs. During
the breeding season _Polypterus_ is unusually active and excitable, and at
this period the anal fin of the male becomes greatly thickened and
enlarged, and has its surface thrown into deep folds between the successive
fin-rays.[574] The use of the modified fin is not known. During his stay at
McCarthy Island, about 160 miles up the River Gambia, Budgett[575] was
fortunate in securing a larva of _P. senegalus_, 1 to 1¼ inches in length,
or only about one-third the length of any larval _Polypterus_ previously
known (Fig. 281). The larva is described as a most beautiful object,
"marked with black stripes on a golden ground, with a conspicuous golden
stripe on each side above the eye, across the spiracle, and along the
dorsal surface of the external gill." The pinnate external or cutaneous
gills were relatively of much greater size than in the considerably more
advanced stage figured elsewhere,[576] and reached half-way to the tail.
The dorsal fin is not divided into finlets, and behind it is continuous
with the caudal, while the anal fin is scarcely distinct from the
{484}lower lobe of the caudal. The fin-rays which support the ventral
portion of the caudal fin are more numerous and longer than those in
relation with the dorsal lobe, and hence at this stage the tail is really
heterocercal.
[Illustration: FIG. 281.—Larva of _Polypterus senegalus_. × 4. Showing its
characteristic attitude when resting on the bottom of an aquarium, and the
large size of the cutaneous gills. (From Budgett.)]
In the genus _Calamichthys_ the body is greatly elongate and Eel-like in
shape. Pelvic fins are absent, and normally there is no suboperculum. The
dorsal finlets are more isolated than in _Polypterus_, and each spine
supports but a single soft ray. Only a single species is known, _C.
calabaricus_[577] (Fig. 282).
[Illustration: FIG. 282.—_Calamichthys calabaricus_. × ⅔. (From a specimen
in the Cambridge University Museum.)]
_Calamichthys_ has a more restricted distribution than _Polypterus_, and is
confined to certain rivers of West Africa. First obtained at Creek Town on
the Old Calabar river, it is now known to occur in the delta of the Niger,
on the coast of Cameroon, and as far south as the river Chiloango,
frequenting the smaller muddy rivers opening into the estuaries.[578] It is
a {485}very agile Fish, swimming like a snake, and subsisting on insects
and crustaceans. The anal fin is enlarged in the male, and the young are
provided with cutaneous gills. _Calamichthys_ may attain a length of nearly
40 cm.
* * * * *
In the remaining Teleostomi (ACTINOPTERYGII) the paired fins are invariably
non-lobate, with abbreviate, multibasal endoskeletal supports. Fin-rays are
the main support of both the median and paired fins. Jugular plates are
usually replaced by branchiostegal rays, but both may co-exist. The
Actinopterygii are the successors of the Crossopterygii in palaeontological
sequence, and when the latter began to decline in Carboniferous and Permian
times, the former, mainly represented by the earlier Chondrostei, had
already become the dominant Fishes of the period.
ORDER II. CHONDROSTEI (ACIPENSEROIDEI).
In these Fishes, the oldest and the most primitive of the Actinopterygii,
the fin-rays of the median fins still continue to retain their primitive
numerical superiority over the radials, and the tail is heterocercal. There
is a single dorsal and an anal fin, which, like the upper lobe of the
caudal fin, are generally provided with fulcra. Pelvic fins abdominal.
Squamation typically rhombic and ganoid. Vertebral column acentrous. So far
as is known the chondrocranium is but little ossified, and the cranial
bones are mainly dermal. The secondary pectoral girdle still includes a
pair of infra-clavicles.
The Chondrostei are first represented in the Lower Devonian by the solitary
Palaeoniscid genus _Cheirolepis_, a contemporary of the earliest
Crossopterygii. They occur throughout the Mesozoic period, except in the
Cretaceous, and also in the Eocene, and while steadily diminishing in
number and variety they gradually approximate to their degenerate and in
some respects highly specialised descendants, the Sturgeons and
Paddle-Fishes of the existing Fish fauna. Of the seven families included in
the group the Palaeoniscidae are the oldest and the most generalised. The
Platysomidae are a specialised offshoot from the Palaeoniscidae, and, if
they are rightly to be considered as Chondrostei, perhaps the same may be
said of the problematic Belonorhynchidae. On {486}the other hand, there are
certain features in the Catopteridae which indicate an approach to Fishes
of an altogether more modern type. Finally, the Chondrosteidae represent a
stage in a career of degeneration, the climax of which is reached by the
modern Polyodontidae and Acipenseridae.
[Illustration: FIG. 283.—_Palaeoniscus macropomus_. Restoration, nearly
one-half nat. size. (From Traquair.)]
[Illustration: FIG. 284.—Outline restoration of the skull and secondary
pectoral girdle of _Palaeoniscus macropomus_. _an_, Angular; _br.r_,
branchio-stegal rays; _cl_, clavicle (cleithrum); _d_, dentary; _d.ect_,
dermal lateral ethmoid; _f_, frontal; _i.cl_, infra-clavicle; _i.op_,
suboperculum; _mx_, maxilla; _n_, nostril; _op_, operculum; _or_, orbit;
_p_, parietal; _p.f_, pectoral fin; _p.mx_, premaxilla; _p.op_,
preoperculum; _p.t_, post-temporal; _s.cl_, supra-clavicle; _s.o_,
circumorbitals; _sq_, squamosal; the single median bone overlying the short
rostrum is probably a dermal mesethmoid, and the one intercalated between
the squamosal and post-temporal a supra-temporal. The dotted lines indicate
sensory canals. (From Traquair.)]
FAM. 1. PALAEONISCIDAE.[579]—Fishes with fusiform bodies, short dorsal and
anal fins, and usually with a complete investment of articulating rhombic,
rarely cycloid, ganoid scales (Fig. 283). Fulcra generally present at the
bases of the median fins, and especially along the dorsal border of the
upper caudal lobe. Ribs are not known to be present. Skull invested by a
very complete series of paired dermal bones, which in number and
disposition conform to the normal Teleostome type (Fig. 284). The secondary
upper jaw includes both premaxillae and large maxillae; and, as a rule,
both the dentary and splenial bones {487}of the lower jaw are dentigerous.
Except for the absence of an interoperculum, the opercular series of bones
is complete, including numerous branchiostegal rays. There is a single
small median jugular plate.
The Palaeoniscidae are remarkable both for their individual and specific
abundance and for their extensive range in time. Represented only by
_Cheirolepis_ in the Middle Old Red Sandstone and Devonian, the family
attained its maximum development in the later Palaeozoic rocks
(Carboniferous and Lower Permian), became rare in the Mesozoic, finally
dwindling away at the close of the Jurassic period. Their geographical
distribution in the past is hardly less remarkable. In various geological
formations they have been found in Great Britain and Ireland, in widely
remote parts of continental Europe, and in North America, South Africa, and
Australia. _Cheirolepis_, _Amblypterus_, _Canobius_, _Phanerosteon_,
_Elonichthys_, _Cryphiolepis_, _Palaeoniscus_, and _Trissolepis_ are
Palaeozoic genera. _Gyrolepis_, _Urolepis_, _Coccolepis_, _Oxygnathus_, and
_Centrolepis_ are characteristic Mesozoic forms.
[Illustration: FIG. 285.—Restoration of _Eurynotus crenatus_. _in.cl_,
Infra-clavicle; _l.l_, lateral line; _orb_, orbit; other reference letters
as in Fig. 284. (From Traquair.)]
FAM. 2. PLATYSOMIDAE.[580]—More or less deep-bodied Fishes, with elongated
dorsal and anal fins, a high head, short jaws, usually armed with bluntly
conical tritoral teeth, and a complete investment of high, narrow, rhombic
scales. They agree with the Palaeoniscidae in their osteology and in most
other essential {488}features, and they flourished in large numbers during
the Carboniferous and Permian periods. _Platysomus_ ranges from the Lower
Carboniferous to the Upper Permian in Great Britain and continental Europe,
and also occurs in the Carboniferous of North America. _Eurynotus_ (Fig.
285), and the singularly deep-bodied _Cheirodus_ (Fig. 286), in which
pelvic fins are unknown, are British Carboniferous genera.
[Illustration: FIG. 286.—Restoration of _Cheirodus granulosus_. _d.ect_,
Dermal lateral ethmoid; _d.eth_, dermal mesethmoid; _d.sp_, either a dermal
sphenotic or a post-orbital bone; _l.l_, lateral line; _orb_, orbit. The
pectoral fin is indicated in dotted outline. Other reference letters as in
Fig. 284. (From Traquair.)]
FAM. 3. BELONORHYNCHIDAE.—The systematic position of these Triassic forms
is very doubtful, and it is by no means clear that they are Chondrostei at
all.
FAM. 4. CATOPTERIDAE.—It is very probable that this widely-distributed
Triassic family is an offshoot from the Palaeoniscidae. It agrees with the
latter in the general character of the head and pectoral girdle and in the
rhombic squamation, but differs from its progenitors and approaches the
more modern Holostei in the semi-heterocercal condition of the tail, and in
the approximate numerical agreement between the fin-rays and radialia of
the dorsal and anal fins.[581]
{489}FAM. 5. CHONDROSTEIDAE.—This family affords an interesting annectant
link between the Palaeoniscidae and their degenerate living representatives
the Polyodontidae and Acipenseridae. They agree with the latter in the
general shape of the body, the growth of a preoral rostrum, and in the
relatively small size of their ventrally-placed and probably protrusible
mouth (Fig. 287). The skin is entirely scaleless, except on the upper lobe
of the caudal fin, where, as in _Polyodon_ and _Acipenser_, the primitive
rhombic squamation and a series of fulcra are retained.
[Illustration: FIG. 287.—Restoration of the skeleton of _Chondrosteus
acipenseroides_. _a.f_, Anal fin; _c.h_, cerato-hyal; _e_, eye; _h.a_,
haemal arches; _hym_, hyomandibular; _j_, jugal; _n.a_, neural arches;
_n.c_, notochord; _n.s_, neural spines; _pc.f_, pectoral fin; _p.f_, pelvic
fin; _s.o_, suborbital; _s.op_, suboperculum; other reference letters as in
Fig. 284. (After Smith Woodward.)]
On the other hand, their relationship to the Palaeoniscidae is indicated by
the general disposition of the dermal bones of the cranial roof, and the
presence of a transverse row of supra-temporals and of an extensive series
of branchiostegal rays (Fig. 288). The family is represented by
_Chondrosteus_[582] from the Lower Lias of Dorset and Leicestershire, and
_Gyrosteus_ from the Upper Lias of Yorkshire. From an evolutionary point of
view it is significant that the Chondrosteidae do not make their appearance
until the Palaeoniscidae are approaching extinction.
The two remaining families, the Polyodontidae and the Acipenseridae, agree
in presenting a remarkable leaven of characters otherwise distinctive of
the typical Elasmobranch, associated with certain primitive features which
they have doubtless inherited from some remote ancestral stock common both
to existing Elasmobranchs and to the other primary groups of Fishes, and
also with others obviously due to degeneration.
{490}[Illustration: FIG. 288.—Lateral view of a restored skull and pectoral
girdle of _Chondrosteus acipenseroides_. _a_, Angular; _br_, branchiostegal
rays; _c.h_, cerato-hyal; _h.m_, hyomandibular; _j_, jugal; _p.f_,
post-frontal; _s.op_, suboperculum; _s.t_, supra-temporal; other reference
letters as in Fig. 284. (After Traquair.)]
The most interesting illustration of the first point is to be found in the
condition of the primitive upper jaw which, especially in the
Polyodontidae, is typically Elasmobranch in the median union of the
palato-quadrate bars beneath the basis cranii, but Teleostome in the
presence of a secondary upper jaw formed by two maxillae. Both families
also agree in possessing an acentrous vertebral column which, if it does so
far resemble that of Teleostomes in being potentially arco-centrous,
nevertheless has a better developed series of distinct inter-dorsal and
inter-ventral cartilages, regularly alternating with only partially bony
basi-dorsals and basi-ventrals, than is to be met with in any other adult
Fishes except Elasmobranchs. Primitive features are apparent in the
presence of spiracles, sometimes associated with pseudobranchs; the
presence in one family (Acipenseridae) of a hyoidean hemibranch supplied
with blood directly from the ventral aorta, and the existence of a
multi-valvular conus arteriosus and an intestinal spiral valve. Finally,
the massive growth of the chondrocranium wholly devoid of cartilage bones,
except in so far as they may be represented by splint-like membrane bones,
the fragmentation of the investing dermal bones, the degeneration of the
opercular skeleton and the loss of branchiostegal rays, and the almost
complete disappearance of the primitive rhombic squamation, are probably to
be regarded {491}as the outcome of a long-continued career of degeneration
from some remote Palaeoniscid ancestor.
FAM. 6. POLYODONTIDAE.—The Polyodontidae are more generalised, and in some
features decidedly more Selachioid than the Acipenseridae. Body fusiform
and apparently scaleless, but the primitive squamation is still represented
by isolated vestigial scales imbedded in the otherwise soft skin, and by a
continuous series of rhombic scales on the upper caudal lobe, which also
has a dorsal fringe of large fulcra.[583] Rostrum exceptionally long,
spatulate or somewhat conical, with a rigid axis and thinner and more
flexible margins. Barbels absent. Mouth wide, not spout-like. Pectoral fins
devoid of spines. Two pairs of membrane-closed vacuities separate the
paired dermal bones of the cranial roof (possibly parietals and frontals)
from the more laterally-placed post-temporals and squamosals, and there are
no median plates posterior to the orbits, nor any representatives of
supra-temporals. A feeble suboperculum is retained in addition to a small
rayed operculum. Hyoidean hemibranch completely suppressed. Two genera only
are known, each with a single species.
[Illustration: FIG. 289.—_Polyodon folium_. _a_, Anus; _f_, fulcra; _n_,
nostrils; _op_, operculum; _sc_, rhombic scales on the upper caudal lobe;
_sp_, left spiracle.]
The Paddle-Fish or Spoon-Bill, _Polyodon folium_ (Fig. 289) inhabits the
rivers of the Southern States of North America, the Mississippi, Ohio, and
Missouri, and their numerous tributary rivers and streams. A Fish of
sluggish habits, _Polyodon_ feeds chiefly on mud and the minute organisms
it contains, the exceptionally long gill-rakers probably forming an
efficient filter to prevent the food particles escaping through the
gill-clefts with the expiratory water current. The singular rostrum is
apparently used for stirring up the mud when feeding, but in view of the
muddy waters the Fish frequents, and the very small size of {492}the eyes,
its value as a tactile organ must not be overlooked. _Polyodon_ may attain
a length of 5 to 6 feet. The time of spawning varies, according to
locality, from March to June. Nothing is known of the development of
_Polyodon_. Young less than 6 to 8 inches in length are unknown, and
specimens of this size are very rarely seen. The jaws are furnished with
minute teeth until the Fish is about half-grown, when they become
edentalous. Caviare is made from the eggs, and the centres at which this
industry is carried on are chiefly situated along the course of the
Mississippi. The second species, _Psephurus gladius_, inhabits the
Yang-tse-Kiang and Hoangho rivers of China, and differs from _Polyodon_ in
the conical shape of its rostrum and the smaller number and larger size of
its fulcra. _Psephurus_ is stated to reach a length of 20 feet. The family
is represented in the Eocene of Wyoming by the genus _Crossopholis_, which
is note-worthy for the retention of trunk scales in the form of small,
somewhat quadrate denticulated discs, arranged in oblique rows.
FAM. 7. ACIPENSERIDAE.—In the Sturgeon family the body is elongate,
cylindrical, and somewhat bulky. Rostrum well developed and often massive,
with a transverse row of simple or branched preoral barbels on its ventral
surface. Mouth small and remarkably protrusible. Jaws devoid of teeth
except in the larvae. As in the preceding family, the primitive rhombic
squamation is confined to the upper lobe of the tail, which, like the
dorsal and anal fins, is furnished with fulcra. Elsewhere the scales are
represented by five longitudinal rows of large bony scutes and by
intervening small scattered ossifications. The anterior dermal ray of the
pectoral fin is stout and spine-like. The dermal bones of the cranial roof
suturally articulate with one another to form a continuous shield,
uninterrupted by lateral vacuities. A median dermal bone in the occipital
region transmits the occipital sensory canal. The opercular series is
represented only by an opercular bone.
The family includes but two genera, _Acipenser_ (Fig. 290) and
_Scaphirhynchus_, and about twenty species, confined to the seas,
estuaries, and rivers of the temperate and north temperate regions of the
northern hemisphere. _Acipenser_ includes the more typical Sturgeons, and
is distinguished by the presence of spiracles, and by the fact that the
longitudinal rows of scutes remain distinct to the base of the caudal fin.
{493}[Illustration: FIG. 290.—The Sterlet (_Acipenser ruthenus_). _o_,
Barbels; _c.f_, caudal fin; _d.f_, dorsal fin; _pct.f_, pectoral fin;
_pv.f_, pelvic fin; _sc_, scutes; _v.f_, ventral or anal fin. (From Parker
and Haswell, after Cuvier.)]
There are probably about fifteen species, but the exact number is
uncertain. Sturgeons are abundant in the Black Sea, the Sea of Azov, the
Caspian, and their tributary rivers, notably the Danube, Don, Dnieper,
Ural, and Volga. They are also present in the rivers and on the coasts of
Northern Europe and of China. Five species occur in North America, on the
Atlantic and Pacific coasts, and in the rivers of these regions as well as
in the Great Lakes.[584] One or two species are almost exclusively
fresh-water, but most Sturgeons are migratory Fishes, living in the sea,
but ascending rivers for spawning. Their food consists of worms, molluscs,
the smaller Fishes and aquatic plants; and in feeding the mouth is
protruded downwards in the form of a cylindrical, spout-like structure and
thrust into the mud. The only species certainly known to frequent the
British coasts is the common Sturgeon (_A. sturio_), which is also found in
the Black Sea and the Mediterranean, and is abundant on the Atlantic coast
of North America from Maine to South Carolina. The species occurs all round
our coasts, more plentifully, perhaps, on the northern and eastern shores.
In the spring and summer the Fish ascends the rivers, often to a
considerable distance. Its presence has been recorded in the Severn, near
Shrewsbury; in the Trent at Nottingham, and also, but not in recent years,
in the Thames above London Bridge.[585] In this country the species is a
"Royal Fish," and by an unrepealed Act of Edward II. it is enacted that
"the King shall have the wreck of the sea throughout the realm, Whales and
Great Sturgeons, except in certain places privileged by the King."[586] If
not so large as some of its Russian relatives, _A. sturio_ often attains a
great size. Even on {494}our own coasts the capture of individuals 8 to 10
feet in length has been recorded. The great Russian Sturgeon (_A. huso_),
which is common in the Black Sea, the Sea of Azov and the Caspian, and in
the rivers flowing into them, is the largest of all the Sturgeons,
individuals weighing 2760 and 3200 pounds having been captured. The Sterlet
(_A. ruthenus_), similarly distributed and often ascending the Danube to
Vienna, is much smaller, rarely exceeding a length of three feet.
[Illustration: FIG. 291.—Larval _Acipenser ruthenus_. _a_, Anus; _b_,
barbels; _e_, eye; _g_, gills; _m_, mouth, with teeth; _ol.o_, olfactory
organ; _op_, operculum; _pt.f_, pectoral fin; _sp_, spiracle. × 10. (From
Kitchen Parker.)]
In Europe _A. sturio_ spawns about July, but in North America (Delaware
river) during May. Small in size, the eggs are produced in enormous
numbers, a single female, it is said, producing about 3,000,000 in one
season. They are invested by a gelatinous sheath, so that they readily
stick to one another or to other objects, and, when deposited, they adhere
in streaks or sheet-like masses to the bed of the river. The young are
hatched very early, about the third or fourth day in _A. sturio_, and in
the Sterlet between the ninth and twelfth, the length of the larva then
varying from 7 to 10 mm. When they are a few days old the larvae closely
resemble those of existing Holostei except that the small opercular folds
leave the gills freely exposed (Fig. 291). A shallow pigmented groove in
front of the mouth apparently represents the sucker of the young _Amia_ and
_Lepidosteus_. Although toothless in the adult, both the Sturgeon and
Sterlet possess vestigial rudimentary, uncalcified, larval teeth, which in
shape resemble the teeth of a Dog-Fish, consisting of a broad base and a
sharp spine.
The Sturgeon is a Fish of considerable economic importance. The flesh is an
article of food, and from the ovaries of certain Russian and American
species thousands of hundredweights of {495}caviare are prepared annually.
Large quantities of isinglass are obtained from the air-bladders, in the
United States and in Russia. The organ is split open and washed; the inner
lining is then stripped off and the bladder dried as rough isinglass.
The second genus, _Scaphirhynchus_, which includes the Shovel-nosed
Sturgeons, differs from _Acipenser_ in the long, flattened, and almost
spatulate shape of the rostrum, the suppression of the spiracles, and the
union of the longitudinal rows of scutes beneath the dorsal fin to form a
scaly armature completely investing the tail. The distribution of the genus
affords an interesting parallel to that of the Polyodontidae. Of the four
species, one (_S. platyrhynchus_) is common in the Mississippi valley and
in the rivers of the Western and Southern States of North America, while
the remaining species, also exclusively fresh-water, frequent the rivers of
Tartary.
The Acipenseridæ are not known to occur earlier than the Tertiary. Scutes,
pectoral spines and fragmentary bones, indistinguishable from the
corresponding parts of existing species, have been recorded from the London
Clay of the Isle of Sheppey (Lower Eocene), and from later Eocene deposits
in the Isle of Wight and Hampshire; and also from the Pliocene of England
(Red Crag of Suffolk) and Virginia.
ORDER III. HOLOSTEI (LEPIDOSTEOIDEI).
The Holostei include a large and somewhat heterogeneous assemblage of
Fishes, most of which are now extinct. As a group they are by no means easy
to define or delimit. Widely separated from the Chondrostei, there is
little evidence of the existence of connecting links between the two
groups, although in some respects the Catopteridae may be regarded as
transitional. On the other side, however, the Holostei shade off almost
imperceptibly into the Malacopterygian Teleostei. In different fossil and
recent Holostei there may be traced the gradual acquisition of the more
special Teleostean characters and the elimination of the more archaic
features of their remote Teleostome ancestors; and in a general sense this
may be taken as the key to the more salient attributes of the group. It is
not suggested that all the families of Holostei are on the direct lines of
Teleostean descent. Some families, like the Eugnathidae and {496}Amiidae,
may possibly occupy this position, but others, such as the Pycnodonts, for
example, seem to be highly specialised and terminal offshoots which have
left no descendants. Of the more generalised features which different
Holostei retain, mention may be made of the prevalence of rhombic scales
which, like the dermal cranial bones, are generally invested by a variously
ornamented coat of ganoin; the presence of fulcra, cheek-plates, post- or
sub-orbital ossicles, and of a complex lower jaw, which includes
dentigerous splenials; and the abdominal position of the pelvic fins. On
the other hand, indication of advancing specialisation in the Teleostean
direction are to be noted in the numerical agreement between the dermal
fin-rays of the median fins and their supporting radialia, and in the
character of the vertebral column. Some Holostei, especially the earlier
forms, are acentrous, but between this primitive condition and the
possession of well-ossified centra, associated with equally bony arcualia,
almost every gradation is to be found. The chondrocranium is more or less
completely replaced by cartilage bones corresponding to those generally
present in Teleosts, while the palato-pterygoid cartilages, likewise
modified by the growth of cartilage bones, separately articulate with the
lateral ethmoid regions instead of meeting in a ventral symphysis beneath
the basis cranii. With rare exceptions (_e.g._ certain Pycnodonts) the
opercular skeleton is complete, and includes branchiostegal rays; and
although a single gular plate is often present, it may be absent in entire
families. Like so many other structures, the tail is in a transitional
state: really heterocercal, but incipiently homocercal, it may be described
as semi-heterocercal. Infra-clavicular plates no longer form part of the
secondary pectoral girdle, their place being taken by cleithra which, as in
most Teleosts, meet in a ventral symphysis.
Indications of transition are not wanting in the squamation in certain
families, and may be seen in the partial or complete replacement of the
rhombic type by thin, imbricated, cycloid scales. Lastly, the soft parts of
the two surviving genera are not without features of similar significance.
A multivalvular conus arteriosus, it is true, is still retained, but the
spiral valve is vestigial, the spiracles are closed, and in the female of
one genus (_Lepidosteus_) the gonoducts are peritoneal tubes, continuous,
as in most Teleosts, with the investments of the ovaries.
{497}The Fishes here included in the Holostei constitute the Protospondyli
and Aetheospondyli of Smith Woodward.[587] In the former group vertebral
centra are either entirely absent, or, if present, their components in the
form of alternating hypo- and pleuro-centra invariably remain distinct in
the tail. The latter group has been instituted for the provisional
reception of two highly specialised families of uncertain relationships,
which differ from the Protospondyli in their higher grade of vertebral
structure, the centra always being complete without any indication of
distinct hypo- and pleuro-centra.
The Holostei first appear in the Permian, where they are represented by a
single genus (_Acentrophorus_). During the Mesozoic period they were
abundant in the Trias, reaching their maximum development and becoming the
dominant Fishes of the period in the Jurassic. In the Cretaceous they began
to decline, and in the Tertiaries became reduced to the two families which
at the present day are the sole survivors of the group.
Of the six families of Protospondyli the Semionotidae are the oldest and
most generalised, and the Macrosemiidae a closely allied group. The
Pycnodontidae are a highly specialised and terminal offshoot. The
Eugnathidae obviously lead to the Amiidae, and from the same stock it is
probable that the Pachycormidae have been derived. The relations of the
Aspidorhynchidae and Lepidosteidae (Aetheospondyli) are extremely doubtful.
That the two families are allied seems probable, but beyond the possibility
of a remote connection with the Protospondyli there is no clue to their
ancestry.
[Illustration: FIG. 292.—Restoration of _Lepidotus minor_. Upper Jurassic,
Dorset. × ⅕. (After Smith Woodward.)]
FAM. 1. SEMIONOTIDAE.—Small-mouthed, fusiform or {498}deep-bodied
Holosteans with rhombic scales, rarely, as in _Aetheolepis_, cycloid in the
caudal region. All the fins possess fulcra. Teeth more or less conical,
with a tendency to become tritoral in certain genera. Jugular plate present
or absent. _Acentrophorus_ (Upper Permian); _Semionotus_ (Trias of England,
Germany, S. Africa, and N. America); _Lepidotus_ (Fig. 292) (Trias of
Germany, Jurassic of Europe and India, Cretaceous of Brazil); the
deep-bodied _Dapedius_ (Lias of Dorset, Fig. 293), and _Aetheolepis_
(Jurassic of New South Wales) are characteristic genera.
[Illustration: FIG. 293.—Restoration of _Dapedius politus_. Lower Jurassic,
Dorset. × ¼. (After Smith Woodward.)]
FAM. 2. MACROSEMIIDAE.—Closely related to the Semionotidae, but with a more
extended dorsal fin. _Macrosemius_ (Upper Jurassic of England, Germany,
France); _Notagogus_ (Upper Jurassic of Naples, Bavaria, France);
_Petalopteryx_ (Upper Cretaceous of Syria).
FAM. 3. PYCNODONTIDAE.—Highly specialised deep-bodied Fishes, with a small
mouth and characteristic grinding or tritoral teeth. Scales rhombic. Fulcra
absent. Dorsal and anal fins long. There is no jugular plate. The family
ranges from the Lower Lias to the Lower Eocene, inclusive. _Mesodon_,
_Mesturus_, _Gyrodus_, and _Microdon_ are Jurassic genera. _Coccodus_ and
_Xenopholis_ occur in the Upper Cretaceous of Syria (Mount Lebanon), and
_Pycnodus_ in various European Eocene formations.
FAM. 4. EUGNATHIDAE.—Large-mouthed, elongate fusiform {499}predaceous
Fishes, with pointed teeth, rhombic scales, short dorsal and anal fins, a
single jugular plate and prominent fulcra. The vertebral centra are
represented by distinct hypo- and pleuro-centra, which may form complete
alternating rings in the tail.
The family first appears in the Trias and ranges throughout the Jurassic
period. _Eugnathus_ (Jurassic) and _Eurycormus_ (Upper Jurassic). _Caturus_
(Fig. 294) has a more extensive range, occurring in the Upper Trias of the
Tyrol and in the Upper Jurassic of England and Bavaria. _Caturus_ and
_Eurycormus_, with their relatively thin, imbricated, cycloid scales, which
have lost the peg-and-socket articulation, form connecting links between
the more typical _Eugnathus_ and the Amiidae.
[Illustration: FIG. 294.—Restoration of _Caturus furcatus_, omitting the
squamation. × 1/11. Upper Jurassic of Bavaria. (From Smith Woodward.)]
FAM. 5. AMIIDAE.—Body fusiform and somewhat compressed. Scales uniformly
thin, cycloid, and imbricated. Single dorsal fin long and low. Anal fin
short. Tail nearly homocercal, with a rounded hinder margin. Fulcra absent
from all the fins. Moderately large conical teeth are present on the
premaxillae, maxillae, palatines and dentaries, and smaller teeth on the
vomers, pterygoids, splenials and parasphenoid. Pre- and post-centra fused
in the trunk, forming complete bony amphicoelous centra, but distinct in
the tail. A single large jugular plate is present. In the solitary living
species the air-bladder is cellular, and its afferent arteries are derived
from a posterior aortic arch. Pyloric caeca absent. Two peculiar comb-like
structures are present on the throat.
The Bow-Fin (_Amia calva_), the sole existing representative of the family,
is abundant in the rivers and lakes of Central and Southern North America,
including the great lakes Huron and {500}Erie. It is a voracious,
carnivorous Fish, preying upon other Fish as well as upon fresh-water
Crustaceans and Insects, very tenacious of life, and of no economic value.
The male is smaller than the female, about 18 inches in length, and is
distinguished by the presence of a round black spot, encircled by a margin
of orange, at the base of the caudal fin (Fig. 295). The female may exceed
24 inches.
[Illustration: FIG. 295.—The Bow-Fin (_Amia calva_). (From a specimen in
the Cambridge University Museum.) × ⅕.]
_Amia_ frequently rises to the surface, especially when the water is foul,
and takes in large mouthfuls of air, and it is probable that the air is
subsequently passed into the spacious cellular air-bladder which acts as a
lung. The breeding season, during which the coloration of the Fish is more
brilliant than at other times, lasts from the beginning of May to June, but
it may begin and end somewhat earlier if the temperature be favourable. The
Fish makes its way from the deeper water, where it has remained sluggish
during the winter, to the spawning ground. This is usually at the swampy
end of a lake where there is an abundance of aquatic herbage intersected by
channels of clear water. There the Fish is said to circle round until the
soft weeds and rootlets are bent and crushed aside, so as to leave an area
having the appearance of a crude form of nest,[588] in which the eggs are
deposited. They may be found in enormous numbers adhering to the leaves and
rootlets of the weedy home. After oviposition the male remains on guard
until the young are hatched out, when they appear to leave the nest in a
body, still under the protection of their watchful parent. At all events a
little later the male has been observed to be accompanied by a swarm of
young fry, which he keeps together by circling round them. The development
of the eggs is remarkably rapid. From the first cleavage of the egg to the
hatching of the embryo the whole {501}process may be completed within from
4 to 8 days. When hatched the larvae are about 5 to 6 mm. long. They
possess a large yolk sac and a preoral sucker for attachment. The pectoral
fins are conspicuous structures before there is any trace of the pelvic
fins.
[Illustration: FIG. 296.—_Amia_ and its nest. (From Bashford Dean.)]
The Amiidae, represented by _Megalurus_,[589] first appear in the Upper
Jurassic of Dorset, France, and Bavaria. In the Cretaceous period the
family is represented by _Amiopsis_. Species of _Amia_ occur in the Eocene
of Europe and North America. In the former area the genus became extinct at
the close of the Lower Miocene period, but in the latter _Amia calva_ still
survives.
FAM. 6. PACHYCORMIDAE.—Large-mouthed, predaceous Amioid Fishes with a more
or less prominent snout and a short dorsal fin. Scales rhombic but thin,
rounded behind, and overlapping, sometimes absent. A single large jugular
plate.
In the earlier forms (_e.g._ _Pachycormus_, Lias) the snout is but slightly
produced, but in _Hypsocormus_ (Upper Jurassic), and {502}especially in
_Protosphyraena_ (Cambridge Upper Greensand and the Cretaceous of Europe
and North America), it becomes greatly elongated and associated with an
exceptionally strong dentition.
[Illustration: FIG. 297.—Restoration of _Hypsocormus insignis_, omitting
the squamation. Upper Jurassic of Bavaria. × ⅛. (From Smith Woodward.)]
FAM. 7. ASPIDORHYNCHIDAE.—Long-bodied Fishes, with a pointed preoral
rostrum, sharp teeth, and deep rhombic scales. Fins small, the dorsal and
anal being remote from the pelvic fins. Fulcra vestigial or absent. Jugular
plates not known.
Two genera only are known. _Aspidorhynchus_ is a Jurassic form.
_Belonostomus_ is Upper Jurassic and Cretaceous. Species of the latter
genus have a very wide distribution (Europe, North and South America, and
Australia).
[Illustration: FIG. 298.—Restoration of _Aspidorhynchus acutirostris_.
Upper Jurassic of Bavaria. × 1/11. (From Smith Woodward.)]
FAM. 8. LEPIDOSTEIDAE.—Body elongate, with a relatively short caudal
region. Tail semi-heterocercal. Scales rhombic, thick, ganoin-coated and
articulated, not vertically elongated on the sides of the body. Dorsal and
anal fins short and remote from the pelvic fins. Median fins with fulcra.
Both the upper and lower jaws more or less elongated, forming a broad and
depressed or a long tapering beak, near the anterior end of which the
nostrils are placed. Eyes small. Vertebral centra well {503}ossified,
opisthocoelous and fused with the neural arches. The metapterygoid bones
have a secondary articulation with the skull.[590] Maxillae segmented into
numerous pieces. Jugular plates absent. Branchiostegal rays reduced to
three on each side. Teeth numerous, slender, and of unequal size. In the
larger teeth the dentine is intricately folded. Pyloric caeca branched and
compacted together into a gland-like mass. Air-bladder cellular, but its
blood is not derived from a posterior aortic arch.
The only known genus is _Lepidosteus_, the existing species of which
frequent the fresh waters of North America.[591] The common or Long-nosed
Gar-Pike (_L. osseus_), remarkable for its long and slender beak, is
generally abundant in the rivers and lakes of the United States from
Vermont to the Rio Grande, and it may reach a length of five feet. The
"Short-nosed Gar" (_L. platystomus_, Günther) has a much shorter and
broader beak, and a similar distribution (Fig. 299). The "Great" or
"Alligator Gar" (_L. viridis_, Günther) has a more southerly habitat,
frequenting the rivers of the Southern States, Northern Mexico, and Cuba.
It is by far the largest species, sometimes reaching a length of 8 to 10
feet.
[Illustration: FIG. 299.—Short-nosed Gar Pike (_Lepidosteus platystomus_.)
× ⅛. (From Bashford Dean, after Goode.)]
_Lepidosteus_ is a voracious Fish, preying upon smaller Fishes, and, except
in the breeding season, it frequents the deeper parts of the rivers or
lakes. The Fish is constantly in the habit of rising to the surface and
emitting bubbles of gas, either through the mouth or by the branchial
clefts, and it is probable that this gas is air which has been previously
swallowed at the surface and passed into the air-bladder. About May
_Lepidosteus_ resorts in large numbers to shallower water, where the
temperature is {504}higher, for the purpose of spawning, each female being
attended by from one to four males.[592] During brief recurring periods of
excitement, accompanied by convulsive lashing movements, the eggs and sperm
are emitted. The eggs are extremely sticky, and adhere tenaciously to the
rocks and stones on which they are deposited. In a few days the embryos
hatch out, and at this stage the larva has a huge mouth surmounted by a
terminal preoral disc, fringed with a row of marginal wart-like suckers
(Fig. 300). The yolk sac is so large as greatly to hamper the movements of
the larva; hence, by means of its suckers, the young _Lepidosteus_ attaches
itself to surrounding objects, and remains almost entirely motionless for
some little time after hatching. Later, about a fortnight after escaping
from the egg, the yolk becomes completely absorbed, the suckers degenerate
and eventually disappear, and the larva, freed from its load of nutritive
reserve, assumes a more active life. After the absorption of the yolk the
larvae of Mosquitos appears to form the exclusive diet of the young
_Lepidosteus_ for some time, but very soon young Fishes are readily
devoured.[593]
[Illustration: FIG. 300.—Larval _Lepidosteus osseus_, 11 mm. long. _a_,
Anus; _a.f_, _c.f_, _d.f_, developing anal, caudal, and dorsal fins; _m_,
mouth; _ol_, olfactory organ; _op_, operculum; _pt.f_, pectoral fin; _s_,
sucker. (From Balfour and N. Parker.)]
_Lepidosteus_ seems to have been abundant in Europe during the Eocene and
Miocene periods, but became extinct before the Pliocene. In North America,
also, the genus dates from the Eocene, and still survives.
ORDER IV. TELEOSTEI.
[For the account of this Order, see pp. 541 f.]
{505}CHAPTER XIX
DIPNEUSTI
SUB-CLASS III. DIPNEUSTI (DIPNOI).
These singularly interesting Fishes are distinguished by their more or less
acutely lobate paired fins and their overlapping cycloid scales, and by the
fact that the bony dermal fin-rays of the median fins are much more
numerous than their supporting radialia. Tail heterocercal or apparently
diphycercal. Nostrils inferior. Vertebral column acentrous. The radialia of
the median fins articulate with the contiguous neural or haemal spines and
agree with them numerically. Skull autostylic. Premaxillae and maxillae
absent, but a secondary lower jaw is represented by certain dermal bones of
which tooth-bearing splenials are the most important, the dentary bones
being absent altogether, or, if present, toothless and small. The cranial
dermal bones include median as well as paired lateral plates, but their
relations to those of other Fishes are very obscure. Two opercular bones
are always present, but branchiostegal rays are unknown. One of the most
important diagnostic features is the dentition. All Dipneusti agree in
possessing large tritoral dental plates supported by the palato-pterygoid
and splenial bones. The secondary pectoral girdle includes only cleithra
and infraclavicles (clavicles). There is a pelvic girdle. Claspers absent.
Of the four families of Dipneusti, two, the Ctenodontidae and the
Uronemidae, are exclusively Palaeozoic. The third, the Ceratodontidae, is
Mesozoic, and still survives. The fourth, the Lepidosirenidae, is known
only by two existing genera.
{506}[Illustration: FIG. 301.—Restoration of _Dipterus valenciennesi_. × ⅕.
(From Traquair.)]
[Illustration: FIG. 302.—Outline restoration of _Phaneropleuron andersoni_.
Upper Devonian. (From Dollo, after Traquair.)]
[Illustration: FIG. 303.—Dental plates of _Dipterus valenciennesi_, nat.
size. A, Upper jaw; B, lower jaw. _n_, Position of the nostrils; _p.p_,
palatine dental plates; _p.pt_, palato-pterygoid bones; _sp_, splenial
teeth. (From Smith Woodward, after Traquair.)]
FAM. 1. CTENODONTIDAE.—Body fusiform. Tail heterocercal or apparently
diphycercal. Excluding the anal fin, which is always distinct, the
remaining median fins are either distinct or continuous. Dental plates
traversed by radiating transverse ridges terminating in rows of conical
denticles (ctenodont dentition, Fig. 303). Vomerine teeth not known.
Cranial bones numerous and small, and, like the squamation, with or without
an investment of ganoin. Jugular plates present or absent.[594] The oldest
genus is _Dipterus_[595] from the Old Red Sandstone of Scotland, where it
is contemporaneous with the earliest Crossopterygii and also with the
oldest known Actinopterygii (_Cheirolepis_). _Phaneropleuron_ (Old Red
Sandstone of Scotland, Upper Devonian of Canada, Fig. 302), _Scaumenacia_
(Upper Devonian of Canada), _Ctenodus_ (Carboniferous of Great Britain and
North {507}America), and _Sagenodus_ (Carboniferous of Great Britain and
Lower Permian of Bohemia) belong to the same family.
FAM. 2. URONEMIDAE.—Body fusiform. Dentition non-ctenodont, consisting of
patches of distinct rounded denticles with a row of basally-confluent
conical denticles along the outer margin of each. Scales thin. All the
median fins are continuous. Tail apparently diphycercal. Cranial dermal
bones as in _Dipterus_. _Uronemus_[596] (Lower Carboniferous of Scotland),
and perhaps _Conchopoma_[597] (Lower Permian of Prussia), are the only
known genera.
The two remaining families possess certain features which cannot be
affirmed to have existed in their extinct allies. Thus, both agree in
exhibiting those striking and, so far as Fishes are concerned, unique
modifications of the air-bladder and vascular system, and the olfactory
organs,[598] which are more or less closely associated with air-breathing
habits and indicate a marked convergence towards the Amphibia. Side by side
with such indications of advancing specialisation in certain directions,
ample evidence of a remote ancestry is to be seen in such primitive
features as the presence of a spiral valve and a multi-valvular conus
arteriosus, and in the short and simple alimentary canal. Of other points
of agreement mention may be made of the absence of jugular plates, the
presence of vomerine teeth, the continuity of all the median fins, and the
apparently diphycercal but probably gephyrocercal character of the tail.
FAM. 3. CERATODONTIDAE.—Body elongated and compressed. Scales large, thin,
non-ganoid, and partially enclosed in dermal pouches. Paired fins biserial.
Chondrocranium complete. Dermal bones wholly devoid of ganoin, reduced in
number but increased in size. Circumorbital bones present. Dental plates
oval, crescentic or triangular, traversed by several radiating enamelled
ridges, terminating in smooth or feebly denticulated biting margins. Lower
jaw with a small toothless dentary on each side. The hyoid arch includes a
small hyomandibular and a hypo-hyal in addition to a cerato-hyal. Branchial
arches five in number and bisegmented. The gills exhibit little evidence of
{508}degeneration. Hyo-branchial cleft open, and associated with a
pseudobranch. The first four branchial arches carry holobranchs.
Air-bladder single. Young not provided with cutaneous gills. Two genera
only are known, the Mesozoic _Ceratodus_ and the still living
_Neoceratodus_. The former genus includes numerous species, for the most
part known only by their dental plates, and has a remarkably wide
distribution in different geological formations. Species occur in the Trias
of England, Germany, India, South Africa (Upper Karoo strata), and also,
but more rarely, in certain Jurassic deposits in England and in
Colorado.[599] _Neoceratodus_ is represented by a solitary species, _N.
forsteri_[600] (Fig. 304, A), which is now restricted to the Burnett and
Mary rivers in Queensland. A somewhat wider distribution of the species in
recent times is indicated by the presence of teeth in the later Tertiary
(alluvial) deposits of Darling Downs, near the borders of New South Wales.
The _Neoceratodus_[601] of the Burnett frequents the comparatively stagnant
pools or water-holes which alternate with shallow runs and are usually full
of water all the year round. In these pools, filled with a rich growth of
aquatic vegetation, and often the favourite haunt of the Platypus
(_Ornithorhynchus_), the Fish is fairly abundant. Inactive and sluggish in
its habits, usually lying motionless on the bottom, the Fish is easily
captured by the natives with hand-nets or baited hooks. _Neoceratodus_
lives on fresh-water Crustaceans, worms, and molluscs, and to obtain them
it crops the luxuriant vegetation of the water-holes much in the same way
that a Polychaet or a Holothurian swallows sand for the sake of the
included nutrient particles. Apparently the air-bladder is a functional
lung at all times, acting in conjunction with the gills. At irregular
intervals the Fish rises to the surface and protrudes its snout in order to
empty its lung and take in fresh air. While doing so the animal makes a
peculiar grunting noise, "spouting" as the local fishermen call it, which
may be heard at night for some distance, and is probably caused by the
forcible expulsion of air through the mouth. Useful as the lung is as a
breathing organ under normal conditions, there can be little doubt that its
value as such is much greater whenever gill-breathing becomes difficult or
impossible.
{509}[Illustration: FIG. 304.—A, _Neoceratodus forsteri_, Queensland; B,
_Protopterus annectens_, Gambia. C, _Lepidosiren paradoxa_, Paraguay. The
lozenge-shaped markings on the surface of B do not represent scales but
areas of the skin outlined by pigment cells. In a fresh specimen the scales
are as completely invisible as in C. (A, from Günther; B and C, from
Lankester.)]
{510}[Illustration: FIG. 305.—A young _Neoceratodus_ four weeks after
hatching. _c_, Cloacal aperture; _l.l_, lateral line; _m_, mouth; _op_,
operculum; _p.f_, pectoral fin. (From Semon.)]
This seems to be the case during the hot season, when the water becomes
foul from the presence of decomposing animal or vegetable matter. Semon
records a striking illustration of this in the case of a partially dried-up
water-hole, in which the water had become so foul that it was full of dead
fishes of various kinds. Fatal as these conditions were to ordinary Fishes,
_Neoceratodus_ not only survived but seemed to be quite healthy and fresh.
Such observations are of exceptional interest. Not only do they afford a
clue to the conditions of life which, in the course of time, probably led
to lung-breathing in _Neoceratodus_, but they also suggest the possibility
that a similar environment has been conducive to the evolution of
air-breathing Vertebrates from gill-breathing and Fish-like progenitors. In
spite of its pulmonary respiration, _Neoceratodus_ more closely resembles
the typical Fishes in its habits than any other Dipneusti. It lives all the
year round in the water. There is no evidence that it ever becomes dried up
in the mud, or passes into a summer sleep in a cocoon, and the
well-developed condition of its gills suggest that these organs play a more
important rôle in breathing than in either _Protopterus_ or _Lepidosiren_.
The Fish is not known to leave the water, and the paired fins, useful no
doubt as paddles, are quite incapable of supporting the bulky body on terra
firma. In fact, when _Neoceratodus_ is taken out of its natural element it
seems to be more helpless than most other Fishes, and, in spite of its
capacity for lung-breathing, soon dies unless kept moist by artificial
means. Spawning takes place from April to November, principally in
September and October. The eggs, invested by a jelly-like coat, secreted by
the oviducal walls, are deposited {511}not in a nest, but singly amongst
aquatic vegetation, and, as they are not adherent, it is probable that they
finally rest on the mud. The early developmental stages exhibit a general
resemblance to those of Amphibia. There is no larval metamorphosis, and at
no period does the young _Neoceratodus_ (Fig. 305) possess cutaneous gills
or a cement organ. The tail is apparently diphycercal from the first, and
the pelvic limbs do not appear until about six weeks after the pectoral
members. It is interesting to note that the dental plates are first
represented by lines or patches of separate denticles (non-ctenodont),
which subsequently fuse basally (ctenodont) before the adult condition is
reached.[602] _Neoceratodus_ is stated to grow to a length of 5 to 6 feet.
FAM. 4. LEPIDOSIRENIDAE.—Body elongate, cylindrical and more or less
Eel-like, with small cycloid scales completely enclosed in the skin. Paired
fins so acutely lobate as to present the appearance of tapering cylindrical
filaments, equally devoid of scales and fin-rays. In a general way the
cranial dermal bones correspond with those of _Neoceratodus_, but the place
of the posterior median bone is taken by a large, gable-like
fronto-parietal bone, situated internal to the head muscles, and in direct
relation with the chondrocranium, which is largely aborted in the
interorbital region. Circumorbital bones absent. Opercular bones much
reduced. Lower jaw without dentary plates. Palatine and splenial dental
plates with three non-denticulate, trenchant ridges. Hyoid arch consists of
cerato-hyals only. Hyoidean cleft closed. Certain of the anterior branchial
arches devoid of branchial filaments; when present the latter are leaf-like
and free. Air-bladder a double lung. There is a larval metamorphosis, and
the young possess cutaneous gills. The family includes two genera,
_Protopterus_ and _Lepidosiren_. In the former genus the paired fins are
either uniserial or they consist of axial mesomeres only; there are six
branchial arches and five clefts; and the larval gills are usually retained
as vestiges throughout life. In _Lepidosiren_ the paired fins are reduced
to the segmented axis, without pre- or post-axial radials. There are five
branchial arches and four clefts, and the cutaneous gills disappear soon
after the larval metamorphosis.
{512}[Illustration: FIG. 306.—Map showing the distribution of the surviving
Dipneusti.]
_Protopterus_ has a wide distribution over the middle portion of the great
African continent, ranging from the river Senegal and the White Nile on the
north to the Congo basin, Lake Tanganyika, and the Zambesi on the south.
Three species are known, _P. annectens_ (Fig. 304), _P. aethiopicus_, and
_P. dolloi_. _Protopterus_[603] is usually found in marshes in the vicinity
of rivers. Voracious in its habits the Fish is mainly carnivorous,
subsisting principally on Frogs, worms, insects, and crustaceans. It is by
no means averse to preying upon its own kind, and if several of these
Fishes are confined in the same aquarium they are apt to give free vent to
their cannibal instincts by biting off the tails or limbs of their fellows.
The missing parts are soon regenerated, but the new members are usually
somewhat abnormal, the tail, for instance, never regaining its original
length, while a new pectoral limb may be bifid or even trifid.[604] The
tail is the principal organ of locomotion, and by its means the Fish is
capable of remarkably quick, agile movements. When slowly moving over the
bottom of an aquarium the paired limbs are observed to move to and fro on
opposite sides alternately in a somewhat bipedal fashion. The limbs are
useless for swimming, although it is possible that they may be helpful in
creeping over the bottom, or in balancing, or as tactile organs.
_Protopterus_ is said to breathe by its lungs as well as by its gills, and
to rise to the surface at short intervals to take in fresh air.
{513}[Illustration: FIG. 307.—Diagram of a torpid _Protopterus_, _in situ_.
_c_, Cocoon; _e_, earth; _f_, funnel leading to the mouth of the Fish; _l_,
lid; _m_, mouth; _m.b_, mouth of the burrow; _t_, tail. (From Newton
Parker.)]
In the dry seasons the marshes in which _Protopterus_ lives become dried
up, and to meet this adverse change in its surroundings the Fish
hibernates, or passes into a summer sleep, until the next rainy season
brings about conditions more favourable to active life. Preparatory to this
summer sleep, and before the ground becomes too hard, the Fish makes its
way into the mud to a depth of about 18 inches, and there coils itself up
in a flask-like enlargement (Fig. 307) at the bottom of the burrow, which
is lined by a capsule of hardened mucus secreted by the glands of the
skin.[605] The mouth of the flask is closed by the capsular wall or lid,
which is perforated by a small aperture. The margins of this aperture are
pushed inwards, so as to form a tubular funnel for insertion between the
lips of the Fish. While encapsuled in its cocoon the Fish is surrounded by
a soft slimy mucus, no doubt for the purpose of keeping the skin moist, and
its lungs are the sole breathing organs, the air passing from the open
mouth of the burrow through the hole in the lid directly to the mouth of
the animal. The nutrition of the dormant Fish is effected by the absorption
of the fat stored up about the kidneys and gonads, somewhat after a fashion
not unknown in the fat-bodies of Insects and the hibernating glands of
Rodents. Even portions of the caudal muscles undergo fatty degeneration,
and thus, in a way which recalls the mode of nutrition of the Salmon during
the breeding season, and of the Tadpole during its metamorphosis, a further
store of nutritive material becomes available for the sustenance of the
Fish during {514}its long summer nap. It is highly probable that the
exceptionally numerous leucocytes act as carriers in the work of
transporting the fatty particles to the different organs and tissues of the
body. The length of the summer sleep naturally varies with the duration of
the dry season, and probably it lasts on an average nearly half the year
(August to December). The cocoons, imbedded in an outward casing of
hardened mud, have often been brought to Europe, and when placed in water
of suitable temperature the long torpid _Protopterus_ escapes from its
prison in a perfectly healthy condition, and resumes its partly branchial
and partly pulmonary mode of breathing. The negroes of the West Coast of
Africa are very partial to these Fishes, which they dig out of the dried
marshes and preserve in their clumps of mud for food. With the advent of
the rainy season, when the marshes become flooded, the _Protopterus_
emerges from its cocoon, and returning to its former active life, soon
enters upon the task of reproducing its kind. The important observations of
Budgett[606] have thrown much light on the curious breeding habits and
development of these Fishes. The Fish makes a nest near the edge of a
swamp. The nest is simply a hole of irregular shape, about a foot in depth,
filled with water and surrounded by long grass (Fig. 308). There is no
lining to the nest, and the eggs are deposited on the bare mud. Until the
eggs are hatched, which occurs about the eighth day, and while the larvae
are in the nest, the male remains on guard, and is apt to bite severely an
incautious intruder. Probably with the view of aerating the eggs the water
is continually lashed about by the tail of the guardian parent. The male
has no trace of the peculiar vascular filaments which adorn the pelvic
limbs of the male _Lepidosiren_ during the breeding season. The early
developmental stages are similar in their main outlines to those of
_Neoceratodus_, but the young are very different. When the young
_Protopterus_ (Fig. 309) is hatched it is provided with a crescentic
glandular sucker or cement-organ, situated on the under side of the head
behind the mouth, by means of which the larva attaches itself to the sides
of the nest, or of the vessel in which it is confined, much in the same way
as the young _Lepidosteus_, and probably for the same reason. It may be
remarked that the sucker agrees in structure, position, and function with
that found in Amphibian tadpoles, but it differs both in position and
structure from its preoral analogue in the young of _Acipenser_, _Amia_,
and _Lepidosteus_.
{515}[Illustration: FIG. 308.—Nest of the _Protopterus_ of the Gambia.
(From Budgett.)]
[Illustration: FIG. 309.—Larval _Protopterus_ on the seventeenth day. _c_,
Cement organ; _c.g_, cutaneous gills; _op_, operculum; _p.l_, pectoral
limb; _pv.l_, pelvic limb; _y.s_, yolk-sac. (From Budgett.)]
A month-old larva has much the aspect of a larval Newt. It has four pairs
of vascular plumose cutaneous gills (Fig. 309), which are retained as
vestiges for a long time or even throughout life, and two pairs of
synchronously-developed limbs. As an interesting instance of a nocturnal
and protective change of colour, it may be mentioned that the dark
chromatophores of the skin of the larva expand in the day-time and the
young Fish becomes darker in colour, and therefore less conspicuous when
seen against a background of black mud or soil. At night the contraction of
the colour-sacs renders the larva more transparent and probably less easily
{516}visible than if opaque. The commencement of pulmonary respiration is
coincident with the degeneration of the cutaneous gills, which takes place
about seven weeks after the deposition of the eggs, and about a month after
the larvae leave the nest. _Protopterus_ is said to attain a length of six
feet.
_Lepidosiren paradoxa_,[607] probably the only species of the genus, is
confined to South America. It occurs along the course of the main Amazon
river, entering some of its larger affluents, such as the Ucayale, the
Madeira, the Rio Negro, and the Tapajóz, and also in the Chaco Boreal to
the west of the Upper Paraguay river. The home of the _Lepidosiren_ (or
"Lolach," as the natives call the Fish) of the Chaco country is to be found
in the wide-spreading marshes and swamps, which for a great part of the
year are almost choked by a luxuriant growth of their own peculiar
vegetation and covered by a floating carpet of surface weeds, with here and
there deeper and clearer water and slow-flowing streams. In the dry season
the water gradually shrinks and the swamps eventually become dried up. Of
sluggish habits, the Fish wriggles slowly about at the bottom of the swamp
like an Eel, using its hind limbs in an irregular bipedal fashion as it
wends its way through the dense network of subaqueous plants. _Lepidosiren_
is not exclusively carnivorous. The large fresh-water snail, _Ampullaria_,
which lives in the swamps in enormous numbers, seems to be its favourite
food; but masses of confervoid Algae are also eaten, and in its earlier
stages it is probable that the Fish is more herbivorous than carnivorous.
The Jacare (_Caiman sclerops_) feeds on _Lepidosiren_, and this fact, and
probably also the cannibal habits of the Fish itself, may explain the
capture of specimens with mutilated tails and regenerated, branched,
pectoral limbs. Like other living Dipneusti, _Lepidosiren_ rises to the
surface to breathe. The intervals are, however, very variable, and no doubt
depend on the relative purity or impurity of the water. Both expiration and
inspiration are said to take place through the mouth. The snout is
protruded on the surface, and the creature expires. After being withdrawn
for a moment the head is again projected, and inspiration takes place
through the partially open lips.
{517}[Illustration: FIG. 310.—Pelvic limb of the male _Lepidosiren_ during
the breeding season. (From Graham Kerr.)]
When the Fish finally sinks a few bubbles of surplus air escape through the
gill-clefts. A nocturnal and protective change of colour, similar to that
described in _Protopterus_, has been observed, and although most strikingly
manifest in the larvae, it also occurs in individuals of older growth. The
flesh is much esteemed as food by the Indians, who wade into the swamps and
transfix the Fishes with spears. During the rainy season the _Lepidosiren_
eats voraciously, and a reserve of fat is stored up in the tissues. Like
its African relative, the Fish ceases to feed on the approach of the dry
season, and eventually hibernates at the dilated extremity of a deep
tubular burrow, the entrance to which is plugged by a small lump of clay
perforated by several round holes. On the rising of the water at the next
rainy season the _Lepidosiren_ pushes out the plug and soon emerges from
its burrow.[608] The breeding season begins soon after the escape of the
Fish. The eggs are deposited in nests in the form of underground burrows
excavated in the black peaty soil at the bottom of the swamp, with an
entrance about 4-5 inches wide. At a depth of about a foot the burrow takes
a horizontal course, its total length varying from 2-5 feet. After the eggs
are laid the male remains to guard them. During the breeding season the
pelvic limbs of the male enlarge and become covered by a rich growth of
highly vascular, blood-red filaments 2-3 inches in length[609] (Fig. 310).
The use of these curious structures is uncertain, but it is not improbable
that they act as accessory gills to enable the male to guard the eggs in
the nest without being forced to resort to the surface to breathe air. The
development is essentially similar to that of _Protopterus_. The larva
(Fig. 311) has four pairs of cutaneous gills in relation with the first,
second, third, and fourth branchial arches, inclusive, the first three
pairs being the homologues of the cutaneous gills of the tailed Amphibia;
and also a cement-organ {518}which disappears shortly before the larval
metamorphosis. At that period the circulation in the cutaneous gills
becomes sluggish, and very soon these organs completely atrophy. About the
same time the hyo-branchial cleft closes up, as in _Protopterus_. The young
_Lepidosiren_ soon begins to breathe air and to become more active and
lively in its habits.[610] The adult may attain a length of four feet.
[Illustration: FIG. 311.—Larval _Lepidosiren_ thirty days after hatching.
_c_, Cement organ; _c.g_, cutaneous gills; _p.l_, pectoral limb; _pv.l_,
pelvic limb. (From Graham Kerr.)]
The relations of the different genera of Dipneusti to one another has been
discussed by Dollo in a remarkably suggestive paper.[611] Until the
publication of this treatise it was generally believed that the modern
Dipneusti, _Neoceratodus_, _Protopterus_, and _Lepidosiren_, especially the
first mentioned, were the most primitive and the more nearly related to the
ancestral stock, while the older types, such as _Dipterus_, were regarded
in the light of highly specialised offshoots. The continuity of the median
fins, the apparently diphycercal character of the tail, and the wholly
cartilaginous condition of the chondrocranium in the modern Dipneusti, were
contrasted with the divided median fins, the heterocercal tail, and the
more extensively ossified chondrocranium of the Palaeozoic forms, and the
belief seemed inevitable. Dollo has shown, however, that there is good
reason for the view that the evolution of the group has taken place in
exactly the opposite direction; that, in fact, the older Dipneusti are the
more archaic, and that their modern representatives have been derived from
them by a sequence of retrogressive changes; or, in other words, the latter
have much the same relation to the former as the degenerate Sturgeons and
Paddle-Fishes to their Palaeozoic ancestors, the Palaeoniscidae. Taking
_Dipterus_, the {519}most ancient of all the known Dipneusti, as a
starting-point, it is possible to select a series of genera which
illustrate the evolution of the group both in structure and in
palaeontological sequence.[612] The series is as follows:—_Dipterus_,
_Scaumenacia_, _Phaneropleuron_, _Uronemus_, _Ceratodus_ (_Neoceratodus_),
_Protopterus_ and _Lepidosiren_. Briefly, the more important structural
modifications observable in the transition from the older to the recent
genera are (_a_) the gradual union of isolated median fins to form a
continuous fin[613]; (_b_) the substitution of a gephyrocercal tail for a
heterocercal[613]; (_c_) the degeneration of the squamation, the thick
ganoid scales of the earlier types being replaced by thin, non-ganoid
scales; (_d_) a reduction in the number of cranial dermal bones and the
loss of their original ganoid investment; (_e_) the suppression of the
jugular plates; and (_f_) a reduction in the size of the opercular bones.
In the last two genera of the series, in which specialisation in some
respects and degeneration in others have reached their maximum, the body no
longer retains the fusiform and more typically Fish-like shape of the older
genera, but, in accordance with Eel-like habits and mode of progression,
has become more or less Eel-like in form.[614] The paired fins are almost
vestigial, while the scales, so deeply insunken in the skin as to be
externally invisible, suggest that the modern Dipneusti are approximating
to a final scaleless as well as to an ultimately limbless condition. As to
the origin of the Dipneusti as a group, it seems reasonable to look for
their ancestors in the early Devonian Crossopterygii with acutely lobate
fins, or, with greater probability, to some still more primitive
Crossopterygian with simple, non-rhizodont teeth, capable by fusion of
giving rise to massive tritoral plates, and involving as a consequence the
substitution of an autostylic for an originally hyostylic skull, and the
suppression of the secondary upper jaw. In fact, when our knowledge of the
development of the surviving Dipneusti and Crossopterygii is more complete,
it is not improbable that the inclusion of the two series of Fishes in
subordinate divisions of the Teleostomi will prove to be amply justified.
The relations of the Dipneusti to the Amphibia are {520}somewhat deceptive,
and it seems improbable that the former group stands in the direct line of
Amphibian descent. In most of their structural features not directly or
remotely associated with air-breathing the Dipneusti are true Fishes, and
the striking resemblances which they present to the Amphibians in the
vascular system and lungs seem to be rather the outcome of physiological
convergence, associated with adaptive and parallel modifications in
structure, and due to the influence of a similar environment, than
indicative of direct ancestral relations. With more reason it may be
inferred that both the Dipneusti and the Amphibia have been derived from
some primitive Crossopterygian ancestor with Elasmobranch tendencies, and
subsequently became modified in certain respects on parallel lines.
{521}CHAPTER XX
APPENDIX TO THE FISHES: PALAEOSPONDYLIDAE—OSTRACODERMI—HETEROSTRACI—
OSTEOSTRACI—ANASPIDA—ANTIARCHI—ARTHRODIRA.
In this chapter it is proposed to treat of certain fossil "Fishes" which,
from our ignorance of much that is essential to a proper estimate of their
true relationships, cannot at present be referred to any of the recognised
primary groups of Fishes.
I. PALAEOSPONDYLIDAE.
The interesting little fossil, _Palaeospondylus gunni_,[615] discovered in
the Lower Old Red Sandstone of Caithness, and first described by Traquair,
represents the calcified endoskeleton of an elongated fish-like organism
about an inch, or not exceeding two inches, in length. The vertebral column
consists of a series of broad, calcified ring-like centra, destitute of
ribs, but possessing neural arches and spines, and in the caudal region
haemal arches and spines in addition. The skull, of which only the ventral
surface is known, has a complete basis cranii, laterally expanded behind by
periotic capsules, and in front by what seem to be bulging olfactory
capsules. Anteriorly, the skull terminates in a ring of calcified cirri.
Behind the skull there are two singular post-occipital plates, one on each
side of the anterior section of the vertebral column. The tail was
apparently furnished with a fringing caudal fin, supported dorsally by the
long forked, neural spines, and below by the much shorter haemal spines.
There is no trace {522}of limbs, limb-girdles, jaws, or branchial arches,
nor any evidence of the existence of scales, denticles, or other
exoskeletal structures.
_Palaeospondylus_ has been regarded as a Cyclostome, a view which derives
its principal support from the resemblance of the cirri-encircled orifice
at the anterior end of the skull to an unpaired nasal or naso-pituitary
aperture, and perhaps some measure of credence from such purely negative
evidence as the apparent want of limbs, and of any structures comparable to
jaws. But even if it be admitted that there is some reason for this view,
it is certain that _Palaeospondylus_ obtained a far higher grade of
specialisation in certain respects than any of the existing Cyclostomata;
the presence of calcified vertebral centra and neural arches is conclusive
on this point.[616] _Palaeospondylus_ has also been compared with a larval
Arthrodiran and with a larval Dipnoid.[617]
[Illustration: FIG. 312.—Restoration of _Palaeospondylus_. The figure shows
the ventral surface of the skull and a lateral view of the vertebral
column. _c_, Calcified cirri; _p.a_, auditory capsule; _t.p_, nasal capsule
(?); _x_, post-occipital plate. (From Parker and Haswell, after Traquair.)]
II. OSTRACODERMI.
The Palaeozoic fish-like forms, which, more as a matter of convenience than
as an expression of real kinship, it has been customary to include in this
group, are amongst the earliest Craniates of which we have any precise
knowledge. {523}Of the three subordinate groups or "Orders" into which they
have usually been divided hitherto, two, the Heterostraci and the
Osteostraci, may, with some show of reason, be considered as related forms,
and although they are characterised by much specialisation on independent
lines, there is yet some evidence of connecting links between the two. The
organisms comprising the third group, the Antiarchi, stand upon a very
different footing, and at present it cannot be said that they are in any
way related to either the Heterostraci or the Osteostraci, or indeed to any
other Craniates whatsoever. The association of the Ostracodermi with the
Cyclostomata, a view which has received more influential support than it
deserves, is based on the presumed absence of jaws and paired fins. The
absence of jaws, which, if present, were almost certainly cartilaginous,
has yet to be proved, and even in the latter group it is by no means
certain that they do not possess structures which, morphologically if not
functionally, are veritable jaws. Nor is it quite certain that the lateral
lobes of some Ostracodermi are neither pectoral flaps nor lateral
fin-folds, to say nothing of the lateral appendages of the Antiarchi. And
to these objections there is the further difficulty that there is
absolutely no evidence that the Ostracodermi are monorhinal in the sense in
which this term is applied to the Cyclostomata.[618] On these grounds it
would seem more in accordance with our present knowledge to regard the
Ostracodermi as an independent group whose exact position in the system has
yet to be determined, including, however, besides the generally accepted
orders Heterostraci and Osteostraci, the recently founded provisional order
Anaspida, but excluding the Antiarchi as a separate and distinct section;
rather than to crystallise in a definite system of classification views
which are either purely conjectural or wholly unjustifiable. Even with this
limitation the Ostracodermi are by no means easy to define, especially if
we include those remarkable shark-like forms from the Upper Silurian rocks
of the south of Scotland which have been so admirably described in the
recent classical memoirs of Dr. Traquair. As a rule, the head and the
anterior part of the body are laterally expanded, and more or less sharply
defined from the rest of the body by prominent postero-lateral angles. The
exoskeleton, which exhibits an extraordinary variety of {524}structure in
the different families, ranges from a uniform covering of dermal denticles
to a condition in which the denticles fuse to form anteriorly a highly
characteristic tessellated or continuous dorsal shield, while posteriorly
they become replaced by a nearly typical rhombic squamation. The tail is
heterocercal. Paired fins of the ordinary piscine type are absent. In some
Ostracodermi it seems probable that the gill-clefts opened into a common
branchial chamber on each side, with a single external aperture, but in
others they may have been ventral. The endoskeleton, jaws, dentition, and
the nostrils are unknown.
ORDER I. HETEROSTRACI.
The exoskeletal structures consist of dentine, or of a tissue resembling
it, never of true bone. The orbits are marginal or lateral in position.
With the exception of a caudal fin there are no median fins.
[Illustration: FIG. 313.—Restored outline of _Lanarkia spinosa_, in the
position in which it occurs as a fossil, the head being flattened and the
tail twisted round so as to appear in profile. On each side a much enlarged
dermal denticle is shown. (From Traquair.)]
FAM. 1. COELOLEPIDAE.[619]—Head and anterior portion of the body flattened
and expanded, with prominent lappet-like postero-lateral lobes, which may
represent continuous lateral fin-folds or a very primitive type of pectoral
fin. Nothing is known of the mouth, but it must have been ventral, nor of
the position of the orbits. Branchial apertures unknown, but transverse
markings on each side of the anterior part of _Thelodus pagei_ may be
indications of a branchial apparatus. The exoskeleton consists of a uniform
covering of hollow pointed spines, devoid of a basal plate and open below
(_Lanarkia_); or of minute shagreen-like tubercles (_Thelodus_). The
tubercles or spines consist of dentine coated by ganoin. Of the only two
known genera, _Thelodus_ is a characteristic Upper Silurian genus {525}from
the Ludlow and Downtonian Beds of Lanarkshire. Detached scales are also
known in the Upper Silurian of England. One species (_Th. pagei_) occurs in
the Lower Old Red Sandstone of Forfarshire, and another (_Th. tulensis_) in
the Upper Devonian of Russia. _Lanarkia_ has only been found in the
Downtonian Beds. None of the Coelolepidae exceed fourteen to fifteen inches
in length.
[Illustration: FIG. 314.—Restored outline of the dorsal surface of
_Drepanaspis gemündenensis_. The tail appears in profile. _m.d_, Median
dorsal plate; _p.l_, postero-lateral plate; _r_, rostral plates. (From
Traquair.)]
FAM. 2. DREPANASPIDAE.—This family[620] affords an interesting transition
to the more highly specialised and carapaced Pteraspidae. The head and
anterior part of the trunk now form a broad oblong shield, rounded in front
and abruptly marked off from the tail by conspicuous rounded angles. The
exoskeleton is no longer uniform. In the caudal region the scattered spines
or shagreen tubercles of the Coelolepids have become transformed into
tuberculated quadrangular scales, which are further differentiated along
the dorsal and ventral margins into ridge scales or fulcra; and from a
similar source by a process of basal fusion a series of larger or smaller
dermal plates are formed as components of large dorsal or ventral shields.
The dorsal shield (Fig. 314) is formed by a large central plate; the
postero-lateral portions by two narrow falciform plates; and the anterior
margin by a series of smaller rostral plates. Between the larger plates the
shield is completed by numerous {526}small polygonal plates. All the plates
are superficially ornamented by small stellate tubercles. The ventral
armature (Fig. 315) is similar to the dorsal. A large mental plate forms
the hinder margin of the transverse slit-like mouth, the anterior limit of
which is defined by the rostral plates already mentioned. Laterally may be
seen a pair of small plates (_x_), each perforated by a small aperture, and
probably indicating the position of some kind of sense-organ. Posteriorly
there is a large median ventral plate, in relation with a pair of anterior
and a pair of posterior ventral plates. The areas between the larger plates
are filled in by numerous small polygonal plates. It is possible that there
is a single external branchial aperture on each side, near the
postero-lateral angle of the shield and behind the posterior ventro-lateral
plate. The sole representative of the family is _Drepanaspis
gemündenensis_, from the Lower Devonian of Gemünden in Rhenish Prussia.
Large examples of this fossil must have exceeded two feet in length.
[Illustration: FIG. 315.—Ventral surface of _Drepanaspis_ (tail in
profile). _a.v.l_, Anterior ventro-lateral plates; _e.l_, external lateral;
_m_, mental plate; _m.v_, mid-ventral; _p.l_, postero-lateral; _p.v.l_,
posterior ventro-lateral; _r_, rostral; _x_, orbit or sensory plate. The
mouth and the supposed cloacal aperture are indicated in black. (From
Traquair.)]
FAM. 3. PSAMMOSTEIDAE.—To this family are referred certain dermal plates
occurring, in a more or less fragmentary condition, in the Old Red
Sandstone and Devonian formations of Great Britain and Russia. In their
size and shape, and in their stellate tubercles, these have been compared
to the dorsal, postero-lateral, and ventral plates of {527}_Drepanaspis_.
That _Psammosteus_ is closely allied to _Drepanaspis_ seems certain, but
for the present the two genera may be retained in separate families.
FAM. 4. PTERASPIDAE.[621]—Until the recent inclusion of the three preceding
families, the Pteraspidae were the only representatives of the
Heterostraci. In the best known genus, _Pteraspis_, there is a marked
reduction in the number of the component plates of the carapace, and only
seven can now be distinguished (Fig. 316): (_a_) a large posterior dorsal
plate, supporting behind a stout spine; (_b_) a conical rostral plate,
covering the preorbital part of the head; (_c_) a pair of small marginal
orbital plates, each with a small aperture, probably for the eye; (_d_) a
pair of posterior lateral or cornual plates, each of which is perforated by
a large oblique foramen, conjecturally an external branchial aperture; and
(_e_) a large ventral plate. There is probably, also, a small median
"parietal," or "pineal," plate, with a pit on its inner surface, situated
between the rostral and posterior dorsal plates. Externally the plates are
sculptured into fine ridges, which in their minute structure and their
crenated free margins are suggestive of linear series of fused denticles.
The tail appears to have been invested by imbricated rhombic scales.
_Pteraspis_ (Lower Old Red Sandstone of Scotland and England, and the Lower
Devonian of Galicia); _Cyathaspis_ (Upper Silurian and Lower Old Red
Sandstone), known only by its dorsal and ventral shields; and _Holaspis_
(Lower Old Red Sandstone of Monmouthshire, and the Upper Silurian of
Pennsylvania), are the only genera.
[Illustration: FIG. 316.—Restored outline of _Pteraspis rostrata_, seen
from the side. The scales on the hinder part of the tail are omitted. (From
Parker and Haswell, after Smith Woodward.)]
ORDER II. OSTEOSTRACI.
While agreeing with the more specialised Heterostraci in the division of
the body into an anterior carapaced portion and a free {528}hinder part
invested by a rhombic squamation, the Osteostraci are distinguished by the
presence of bone as a histological component of the dermal hard parts; by
the position of the orbits, which, instead of being marginal in position,
are close together on the dorsal aspect of the carapace; and by the
possession of a median dorsal fin.
FAM. 1. ATELEASPIDAE.[622]—The general shape of the body is much the same
as in the Coelolepidae, but the exoskeleton consists of numerous polygonal
tuberculated plates in front of the postero-lateral lobes, and of
sculptured rhombic scales behind. A pair of crescentic markings, placed
close together about the middle of the dorsal surface of the head, probably
indicate the outer margins of orbital recesses (Fig. 317). The only species
at present known (_Ateleaspis tessellata_) occurs in the Downtonian beds.
[Illustration: FIG. 317.—Outline sketch of _Ateleaspis tessellata_. The
crescentic markings indicate the position of the supposed orbits. (From
Traquair.)]
[Illustration: FIG. 318.—_Cephalaspis murchisoni_. Upper Silurian and Lower
Old Red Sandstone, _op_, Operculum(?). (From Smith Woodward.)]
FAM. 2. CEPHALASPIDAE.[623]—In this family the dorsal shield is rounded in
front, strongly arched above, with its postero-lateral angles produced into
highly characteristic cornua (Fig. 318). The shield consists of a single
piece, but as the outer surface is ornamented by small tubercles arranged
in polygonal areas, it is probable that it has been formed by the basal
fusion of {529}numerous primitively distinct polygonal plates (Fig. 319,
A). Between the orbits there is a separately calcified but fixed plate,
which bears a hollow prominence, probably for the reception of a parietal
organ. In some genera certain of the anterior dorsal and ventral scales of
the trunk fuse into a continuous plate. Internally to the postero-lateral
cornua the middle layer of the shield is prolonged backwards into a pair of
singular flap-like lobes, which have been variously interpreted as
corresponding to the lateral lobes of the Coelolepidae, to pectoral fins,
or to opercula. The scales of the trunk and tail are rhombic and
imbricated; on the sides of the body they are remarkably high and narrow.
[Illustration: FIG. 319.—The dorsal shield of _Cephalaspis lyelli_ (A), and
an outline sketch of the dorsal shield of _Eukeraspis pustulifera_ (B).
_c_, Postero-lateral cornu; _d_, posterior angle; _i.p_, interorbital
prominence; _o_, orbit; _o.p_, orbital prominence; _p.s_, posterior spine;
_p.v_, postorbital valley. (From Lankester.)]
The best known genus is _Cephalaspis_. The earliest remains are found in
the Ludlow Tilestones. The genus is also represented in the Ledbury Passage
Beds, the Lower Old Red Sandstone of Scotland, and the Upper and Lower
Devonian of Canada. Most of the species are of small size, but _C.
magnifica_,[624] from the Caithness Flagstones, the largest of all the
Cephalaspids, has a shield 8½ inches long, and 12 inches across the widest
part. _Auchenaspis_ occurs in the Ludlow Tilestones and the Ledbury Passage
Beds, and also in the Upper Silurian of the Isle of Oesel in the
{530}Baltic. Another genus, _Didymaspis_, has been found in the Lower Old
Red Sandstone of Ledbury.
FAM. 3. TREMATASPIDAE.—The interorbital plate is free, and hence it is
often lost in the fossils. Several species of _Tremataspis_ occur in the
Upper Silurian of the Isle of Oesel.
As regards the origin and mutual relationships of the different families
comprising the Heterostraci, it has been urged with great force by Dr.
Traquair[625] that they constitute a natural sequence of forms, beginning
with organisms whose Elasmobranch ancestry is extremely probable, and
leading to highly-specialised types, which, considered by themselves,
possess little to justify any conclusions whatever as to their origin or
kinship. The Coelolepidae form the starting-point, and in the light of
their exoskeleton of dermal denticles, their derivation from some primitive
Elasmobranch prototype seems a reasonable inference.[626] From the
Coelolepids the path of specialisation through the Drepanaspidae and
Psammosteidae to the Pteraspidae is marked (i.) by the basal concrescence
of isolated denticles to form, first, numerous small polygonal plates, and
then larger and less numerous plates, as the constituent elements of a
characteristic dorsal shield, leaving, however, the denticles of the rest
of the body to become converted into a rhombic squamation; (ii.) by
modifications in the "lateral fin-lobes," which may become enclosed in the
developing dermal armour (_e.g._ _Drepanaspis_), or cease to be
recognisable (_e.g._ _Pteraspis_). The affinities of the Osteostraci are
very obscure, and their inclusion with the Heterostraci in the same group
(Ostracodermi) has hitherto rested mainly on such negative evidence as the
supposed absence of paired limbs, jaws, and teeth; in fact, it has been
affirmed that "there is absolutely no reason for regarding _Cephalaspis_ as
allied to _Pteraspis_ beyond that the two genera occur in the same
rocks."[627] It is possible, however, that in _Ateleaspis_ we have an
annectent form, which in some measure combines the structural peculiarities
of the two groups. That this singular genus belongs to the Osteostraci is
proved by the presence of bone lacunae in its dermal hard parts, a
conclusion which is strengthened by the apparently dorsal position of the
orbits and the presence of a dorsal fin. On the {531}other hand, its close
resemblance to the Coelolepids in the general contour of its
laterally-lobed body, and the probability that its mosaic and tuberculated
head-shield has been formed by the concrescence of Coelolepid denticles, is
at least significant of a relationship to the more primitive Heterostraci.
Little can be conjectured as to the habits of these ancient "Fishes." The
form and regional proportions of the body, which in some respects often
remind one of organisms so diverse as a King Crab, or a Loricaroid Teleost
(such as _Liposarcus_), are strongly suggestive of a grovelling,
bottom-feeding, sluggish habit of life, in sharp contrast to the more
active and predaceous Fishes whose appearance is coincident with the
extinction of the Ostracodermi at the close of the Devonian period. Habits
such as these may well be associated with much structural degeneration,
even, it may be, with the loss of paired fins, and hence it is not
altogether improbable that the Ostracodermi are outcasts from the
Elasmobranchs, a degenerate race which has sought safety in a sequestered
life and a coat of mail.
ORDER III. ANASPIDA.
This group has been instituted by Traquair[628] for the provisional
reception of two remarkable genera, which, owing to the absence of precise
knowledge of the histology of their exoskeletal structures, cannot at
present be referred either to the Heterostraci or the Osteostraci, and for
which, as their discoverer remarks, no place can be found in the system
unless they are admitted to the Ostracodermi.
FAM. 1. BIRKENIIDAE.—Body fusiform and fish-like. Head bluntly rounded,
without a cranial shield. Caudal fin bilobate and heterocercal A median row
of scales with recurved spines arranged along the ventral surface. Orbits,
jaws, teeth, paired fins, and endoskeleton unknown.
In _Birkenia_ (Fig. 320) the body is invested by longitudinal rows of
narrow scales arranged in oblique transverse rows, which are replaced on
the head by much smaller, peculiarly disposed, spindle-shaped scutes. On
the side of the hinder part of the head there is an oblique row of small
apertures, possibly branchial. A small remote dorsal fin, invested by the
trunk scales, is present. {532}_Birkenia elegans_, the only species known,
does not exceed 3½ inches in length. Less is known about the second genus,
_Lasanius_, of which there are two species. Except for the mid-ventral
series of spiny scutes, and a row of slender, parallel, rod-like
structures, the body appears to have been naked (Fig. 321). The two genera
belong to the remarkable series of fossil Fishes from the Silurian rocks of
Lanarkshire. Rare in the Ludlow series, _Birkenia_ is by far the most
common of the Fishes of the over-lying Downtonian Beds. _Lasanius_ is
confined to the latter horizon. _Euphanerops_, from the Upper Devonian of
Canada, is probably related to this family, but lateral branchial apertures
are not known.[629]
[Illustration: FIG. 320.—Restored outline of _Birkenia elegans_ Traq.,
one-half larger than natural size. _b.a_, Branchial aperture; _d_, dorsal
fin. (From Traquair.)]
[Illustration: FIG. 321.—Restored outline of _Lasanius problematicus_,
enlarged, _r_, Post-cephalic rods; _r′_, row of small spine-like scutes;
_v.s_, mid-ventral spine-like scales. (From Traquair.)]
III. ANTIARCHI.
The organisms comprising this group[630] resemble the Ostracodermi in
possessing a well-developed carapace of bony plates and a heterocercal
tail, as well as in many of the purely negative features which are
characteristic of the latter group.
{533}[Illustration: FIG. 322.—Restored outline of _Pterichthys milleri_.
The upper figure represents a dorsal view, and the lower a lateral view.
The dotted lines indicate the course of the lateral line system. _a.d.l_,
Antero-dorso-lateral; _ag_, angular; _a.m.d_, anterior median dorsal;
_a.v.l_, anterior ventro-lateral; _e.l_, extra-lateral or operculum; _l_,
lateral; _l.occ_, lateral occipital; _m_, median or interorbital plate;
_m.occ_, median occipital; _o_, orbit; _p.d.l_, posterior dorso-lateral;
_p.m_, pre-median; _p.m.d_, posterior median dorsal; _pt.m_, post-median;
_p.v.l_, posterior ventro-lateral. ——, Plates investing the limbs: _c_,
central; _d.a_, dorsal anconeal; _d.ar_, dorsal articular; _e.m_, external
marginal; _i.m_, internal marginal; _m.m_, marginals; _t_, terminal. (From
Traquair.)]
The remarkable dorsal shield is divided into a small cephalic portion and a
much larger hinder part investing the greater part of the trunk, both of
which are strongly arched above and flattened ventrally, with a movable
articulation between the two. The cephalic shield is formed by numerous
symmetrically-disposed tuberculated plates, suturally connected with one
another, and, like the other exoskeletal structures, containing bone
lacunae (Fig. 322).[631] The orbits are close together, near the middle of
the dorsal surface, and between them there is a small median interorbital
plate, with a deep pit on its inner surface, possibly for a parietal organ.
A small lateral plate (_e.l._), evidently free behind, suggests the
presence of an operculum. Nothing is certainly known about the jaws or the
nostrils. The mouth is situated just behind the anterior margin of the
cephalic shield on the ventral surface, and in front of it there are two
plates, {534}which in _Bothriolepis canadensis_ have their oral margins
fringed by small "denticles"; it is possible that these plates represent
the components of a secondary upper jaw. The dorsal armature of the trunk
is shown in Fig. 322. Ventrally it is completed by a pair of anterior
ventro-lateral plates and a pair of posterior ventro-lateral plates with a
small median plate between the two pairs. Articulating with the anterior
ventro-lateral plates by means of a complex hinge joint there is a pair of
pectoral appendages of a kind entirely without parallel in any other
vertebrated animals. Each appendage is completely encased by numerous
suturally connected plates, and about the middle of its length there is a
second movable joint. The appendages are hollow, and their cavities
probably contained the muscles by which the limbs were moved, and the
blood-vessels and nerves for their nutrition and innervation. A lateral
line system of the normal type is present in _Pterichthys_, consisting of a
lateral groove along the side of the trunk, and of supra-orbital and
infra-orbital grooves, and post-temporal and infra-orbital commissures, on
the head. The free portion of the body and the tail are invested by
imbricated and finely tuberculated scales, which form fulcra in front of
and behind the small dorsal fin. There are no pelvic fins. The caudal fin
is heterocercal.
FAM. 1. ASTEROLEPIDAE.—The best known genera are _Pterichthys_ from the
Lower Old Red Sandstone of Scotland and the Devonian of Eifel, and
_Bothriolepis_, a more widely distributed genus which occurs in the Upper
Old Red of Scotland and Shropshire, and in the Upper Devonian of Russia and
Canada. Two other genera, _Asterolepis_ and _Microbrachius_, are also found
in the Old Red Sandstone of Scotland.[632]
Beyond an uncertain and shadowy relationship to the Ostracodermi, and
perhaps some points of resemblance to the Arthrodira, the Antiarchi stand
alone among Craniates. Nothing is known of their origin; no intermediate
forms link them to any other groups, and the high specialisation they have
attained is sufficient to negative any idea that they can "be credited with
any share in the evolution of the Fishes of more recent periods."
{535}IV. ARTHRODIRA
This group has been instituted for the reception of a number of remarkable
armoured Fishes of uncertain relationships which flourished in Europe
during the Devonian and Old Red Sandstone periods, and in North America
from the Devonian to the Lower Carboniferous. The head (_e.g._ in
_Coccosteus_)[633] is invested dorsally by a series of median and lateral
symmetrically-disposed tuberculated plates (Fig. 323). Two of the lateral
plates are notched for the orbits, and between them there is an
interorbital plate which either has a pit on its inner surface or is
perforated by an open funnel, as in _Dinichthys_, possibly for a parietal
or a pineal organ. Some of the bones present some analogy, to say the
least, to certain of the dermal bones of a typical Teleostome, apparently
representing such elements as paired parietals and frontals, a dermal
mesethmoid, and toothless premaxillae and maxillae (Fig. 324, A). As in the
Antiarchi, the anterior portion of the trunk is also armoured, above by a
dorsal shield, formed by median and lateral plates, and below by a
similarly constructed ventral shield (Fig. 324, B). A huge joint connects
the head and trunk shields: hence the term Arthrodira or "joint-neck." The
rest of the body is naked.
[Illustration: FIG. 323.—Restoration of _Coccosteus decipiens_. Old Red
Sandstone of Scotland. × ¼. _A_, Articulation of the cephalic and trunk
shields; _DB_ and _DR_, radials of the dorsal fin; _H_, haemal arches and
spines; _MC_, sensory canals; _N_, neural arches and spines; _NT_,
notochord; _U_, median plate; _VB_, basipterygium; _VR_, radialia of the
pelvic fin. (From Parker and Haswell, after Bashford Dean and Smith
Woodward.)]
Pectoral fins are unknown, but pelvic fins, each supported by a stout basal
plate or basipterygium, and with traces of radials, are present. There is a
small dorsal fin. Little is known of the primary cranium, but in the trunk
and tail it is evident {536}that there are well-developed and partially
calcified neural and haemal arches associated with a persistent notochord.
It is possible that the skull is autostylic. Gill-arches are not known. A
pair of plates (Fig. 324, A, _j_) at the postero-lateral angles of the
cephalic shield may perhaps be opercula. The teeth are conical. Those in
the upper jaw are supported by two pairs of plates, probably vomers and
palatines. In the lower jaw there are two series of teeth, one in front
near the symphysis, and the other behind, supported by a single bone in
each ramus. There is a well-developed lateral line system, indicated by
surface markings on the head and trunk shields.
[Illustration: FIG. 324.—Dorsal view of the cephalic and trunk shields of
_Coccosteus_ (A); and a view of the ventral part of the trunk armour (B).
_a.d.l_, Anterior dorso-lateral; _a.l_, antero-lateral; _a.m.v_, anterior
median ventral; _a.v.l_, anterior ventro-lateral; _c_, central; _e.o_,
external occipital; _i.l_, internal lateral; _j_, jugal; _m_, marginal;
_m.d_, median dorsal; _m.e_, dermal mesethmoid; _m.o_, median occipital;
_m.v_, median ventral; _mx_, maxilla; _n_, nasal aperture; _o_, orbit; _p_,
pineal plate; _p.d.l_, posterior dorso-lateral; _p.mx_, premaxilla; _p.o_,
preorbital; _pt.o_, post-orbital; _p.v.l_, posterior ventro-lateral. (From
Traquair.)]
FAM. 1. COCCOSTEIDAE.—_Coccosteus_ occurs in the Devonian of Europe and
North America, and includes species of relatively small size, not exceeding
half a metre in length. _C. decipiens_, the best known species, is a
characteristic fossil in the Old Red Sandstone of Scotland.
_Phlyctaenaspis_[634] is found in the Lower Devonian of Canada, England,
and Poland. A larger Arthrodiran, with slender toothless jaws,
_Homosteus_,[635] is met with in the Lower Old Red Sandstone of the North
of Scotland, and in the Devonian of Germany and Russia. The Old World
Arthrodira must yield, however, to those of the New World for variety in
size and shape, and in the character of their dentition.[636] {537}Some of
the North American genera (_e.g. Dinichthys_) probably attained a length of
ten feet, or even, as in _Titanichthys_, a much greater size. Some are
fusiform in shape, but _Mylostoma_ is flattened and Ray-like, and, judging
from the dentition, their food and habits must have been equally varied.
_Mylostoma_ has tritoral plates not unlike those of _Neoceratodus_ or
_Chimaera_. In others the teeth are single, and conical or pointed; in
_Titanichthys_ the front teeth in both jaws are beak-shaped structures. It
is highly probable that _Titanichthys_, _Mylostoma_, and perhaps other
genera, are types of distinct families.
The Arthrodira have been regarded as armoured Dipneusti, a view which is
mainly based on their supposed autostylism and the nature of the dentition.
But this autostylism has yet to be verified, and, if proved, the
possibility that it may be a secondary feature, associated with the
evolution of a peculiar dentition, must not be forgotten. Much more may be
said for their claim to be regarded as a highly specialised race of
primitive Teleostomi. Besides a well-developed lower jaw, bones comparable
to the elements of a secondary upper jaw are known, and in a general way
the disposition of the cranial roofing bones, and the arrangement of the
endoskeletal elements of the pelvic fins, tend to conform to the normal
Teleostome type. In fact, Dr. Traquair has expressed the opinion that the
Arthrodira are Teleostomi and Actinopterygii.[637]
TELEOSTEI
(SYSTEMATIC PART)
BY
G. A. BOULENGER, F.R.S., V.P.Z.S.
Of the British Museum (Natural History)
{541}CHAPTER XXI
TELEOSTEI: GENERAL CHARACTERS—MALACOPTERYGII—OSTARIOPHYSI
ORDER IV. TELEOSTEI.[638]
As stated above (p. 495), the Holostean Ganoids pass very gradually into
the Teleosteans, the lower groups of which appear to have been directly
derived from them. The precise definition of the Order Teleostei, as
compared with the Ganoid Order Holostei, is a matter of some difficulty.
The most important character appears to be the presence of an ossified
supraoccipital bone.[639] Remnants of primitive characters, such as ganoid
scales, fulcra, rudiments of a splenial bone, spiral valve to the
intestine, multivalvular conus arteriosus, are still found in some lower
Teleosteans, but no longer in that combination which serves to define the
preceding order. Although _Albula_ is exceptional among all Teleosteans in
having two transverse series of valves to the bulbus arteriosus instead of
one, no Ganoid has fewer than three.
The first remains of Teleosteans appear scantily in the Upper Trias, and it
is not before we reach the Upper Cretaceous that they assume preponderance
over other Teleostomes; whilst in the Upper Eocene they have already
attained a development and variety of types comparable to their present
condition. Out of some 12,000 well-established species of Fishes known to
exist at {542}the present day, about 11,500 belong to this order. The
classification of such an array of forms is, of course, a matter of great
difficulty, and gives scope for much difference of opinion among those who
have attempted to grapple with the subject. It is now recognised that the
study of the skeleton affords the safest guide to a natural arrangement of
the families and higher divisions. Much has been done in this line by Cope,
Gill, Sagemehl, A. S. Woodward, and Jordan and his pupils; but the
osteology of many important types still remains unknown. For some years a
large number of skeletons have been prepared in the British Museum with the
object of settling open questions, and this material has enabled me to draw
up a scheme of classification which, whatever its defects, and however
provisional, I feel sure is on the whole an improvement on those hitherto
proposed, and especially on that generally in use in this country. The
latter was, to a great extent, based on physiological principles; the
present aims at being phylogenetic. In its preparation I have derived great
benefit from the labours of the authors quoted above, but have endeavoured
in every instance to verify their statements on a larger osteological
material than appears to have been available to them. I have also had the
advantage of the criticism, on many points, of my young colleague, Mr. C.
Tate Regan, who has himself endeavoured to settle some important questions
of classification.[640]
The Order Teleostei is divided into thirteen sub-orders, the probable
relations of which are expressed in the following diagram:—
11. Opisthomi. 13. Plectognathi. 12. Pediculati. –––+
| | | |
+––––––––––––––––––+––––––––––––––––––––+ |
| |
9. Anacanthini. 10. Acanthopterygii. 8. Percesoces. |
| | | |
+––––––––––––––––––+––––––––––––––––––––+ |––Teleostei.
| |
7. Catosteomi. 5. Haplomi. 6. Heteromi. |
| | | |
+––––––––––––––––––+––––––––––––––––––––+ |
| | |
| 4. Apodes. |
| 3. Symbranchii. |
| |
1. Malacopterygii. 2. Ostariophysi. |
| | –––+
+––––––––––––––––––––––+
|
Ganoidei Holostei.
{543}In the classification of Günther, which has been generally in use in
this country for the last thirty years, the Teleosts were divided into six
principal groups, of ordinal rank: I. Acanthopterygii; II. Acanthopterygii
Pharyngognathi; III. Anacanthini; IV. Physostomi; V. Lophobranchii; VI.
Plectognathi. Group I. corresponds to Sub-Order 6 (part), 7 (part), 8
(part), 10 (part), 11 and 12 of the present work; Group II. to Sub-Order 10
(part); Group III. to Sub-Order 9 and 10 (part); Group IV. to Sub-Order 1,
2, 3, 4, 5, 6 (part), and 8 (part); Group V. to Sub-Order 7 (part); and
Group VI. to Sub-Order 13.
SUB-ORDER 1. MALACOPTERYGII.
Air-bladder, if present, communicating with the digestive tract by a duct.
Opercle well developed. Pectoral arch suspended from the skull;
mesocoracoid arch present.[641] Fins without spines, the ventrals
abdominal, if present. Anterior vertebrae distinct, without Weberian
ossicles.
This sub-order, which corresponds to the Isospondyli and Scyphophori of
Cope and to a part of the Isospondyli of A. S. Woodward, embraces the most
generalised of the Teleosts, and is intimately connected with the Ganoids
by the fossil forms which are placed at the base of the series of families.
The physostomous condition of the air-bladder, the connexion of the
pectoral arch with the skull, the presence of the mesocoracoid arch, the
backward position of the many-rayed ventral fins, the normal condition of
the anterior vertebrae, the absence of true spines to the fins, and the
separation of the supraoccipital bone from the frontals by the parietals,
are primitive characters which among the Teleosts occur combined in some
families of this suborder only. The mesocoracoid arch is retained by the
Ostariophysi, which differ in the remarkably modified condition of the
anterior vertebrae, but it disappears in all other Teleosts, which
gradually acquire a more forward position of the ventral fins and a
reduction in the number of their rays, develop spines in the vertical fins,
and lose the communication of the air-bladder with the outside.
The Malacopterygii may be divided into twenty-one families, the characters
of which are contrasted in the following synopsis:—
I. Fins fringed with fulcra, or scales coated with ganoin; {544}
notochord
usually continuous through the vertebrae.
Vertebral centra not more than rings; fins with fulcra; scales
rhombic, united by peg-and-socket joints 1. _Pholidophoridae_.†
Vertebral centra not more than rings; fins with fulcra; scales
cycloid 2. _Archaeomaenidae_.†
Vertebral centra complete or with minute perforation; fins with
fulcra; scales cycloid 3. _Oligopleuridae_.†
Vertebral centra nearly complete, but with perforation; no fulcra;
scales cycloid 4. _Leptolepididae_.†
II. Fins without fulcra; scales without ganoin; vertebral centra
complete.
A. Supraoccipital separated from the frontals by the parietals or
upper surface of skull; no adipose fin.
1. Ventral fins with 10 to 16 rays.
An intergular bone; parasphenoid narrow 5. _Elopidae_.
No intergular bone; parasphenoid very broad 6. _Albulidae_.
2. Ventrals with not more than 7 rays.
a. Supratemporal very large, plate-like.
Praemaxillary single, its posterior extremity free from the
maxillary; symplectic absent; basis cranii simple
7. _Mormyridae_.
Praemaxillary paired, its posterior extremity firmly attached
to the maxillary; symplectic present; basis cranii double
8. _Hyodontidae_.
b. Supratemporal small; maxillary firmly attached to posterior
extremity of praemaxillary.
Praemaxillary paired; a large hole on each side of the skull,
between the post-frontal and the squamosal; basis cranii
double; suboperculum absent; ribs sessile
9. _Notopteridae_.
Praemaxillary paired; basis cranii simple; suboperculum reduced;
ribs inserted on parapophyses 10. _Osteoglossidae_.
Praemaxillary single; basis cranii simple; suboperculum and
interoperculum absent; ribs inserted on parapophyses
11. _Pantodontidae_.
c. Supratemporal small; maxillary movable; ribs sessile; ventral
fins below the pectorals 12. _Ctenothrissidae_.†
B. Supraoccipital separating parietals, or adipose fin present.
1. Interoperculum enormous; symplectic absent; basis cranii simple
13. _Phractolaemidae_.
2. Interoperculum normal; symplectic present; basis cranii double.
a. Teeth in sockets; maxillary firmly attached to praemaxillary.
Symplectic exposed 14. _Saurodontidae_.†
b. Teeth not in sockets.
Symplectic concealed between the quadrate and the hyomandibular
15. _Chirocentridae_.
Postclavicle on outer side of clavicle; no adipose fin
16. _Clupeidae_.
Postclavicle on inner side of clavicle; an adipose dorsal fin
17. _Salmonidae_.
Postclavicle absent; no adipose fin 18. _Alepocephalidae_.
3. Interoperculum normal; basis cranii simple. {545}
Maxillary large, toothed; praecaudal vertebrae without well-marked
parapophyses; scales cycloid or absent; adipose dorsal fin
present or absent 19. _Stomiatidae_.
Mouth small, toothless; vertebrae with strong parapophyses; head
and body covered with spiny scales 20. _Gonorhynchidae_.
Mouth small, toothless; no symplectic; head and body naked
21. _Cromeriidae_.
† This sign indicates that the group is represented by fossil forms only.
FAM. 1. PHOLIDOPHORIDAE.—Margin of the upper jaw formed by the
praemaxillaries and the maxillaries, the latter large and loosely attached;
teeth small and conical. Parietal bones separating the supraoccipital from
the frontals; opercular bones well developed. Vertebral centra never
advanced beyond the annular stage; ribs delicate; no fused or expanded
haemal arches at the base of the tail; no epipleurals or epineurals.
Fin-fulcra present, but usually small; dorsal and anal fins small, the
former above or behind the ventral fins, which are small or may be absent.
Scales ganoid, rhombic, those on the flanks united by peg-and-socket
joints.
This family, which appears to me to be related to the Dapediidae, is
provisionally placed here by A. S. Woodward on account of its resemblance
to the Leptolepididae, but it is not yet quite clear that the mandible was
destitute of splenial and coronoid elements, while the bones at the base of
the pectoral fin have not hitherto been observed. The principal genera are
_Pholidophorus_, ranging from the Upper Trias to the Purbeck;
_Thoracopterus_, from the Upper Trias; and _Pleuropholis_, from the Upper
Jurassic. The species of _Pholidophorus_ are very numerous in the Jurassic
period, and Woodward has observed that the scales of the later species are
more elaborately ornamented than those of earlier date.
FAM. 2. ARCHAEOMAENIDAE.—Distinguished from the preceding by the thin,
cycloid scales. Conspicuous obtuse ridge-scales are present along the
dorsal and ventral lines. _Archaeomenes_, from the Jurassic (?) of New
South Wales.
FAM. 3. OLIGOPLEURIDAE.—Characters as in Pholidophoridae, but scales
cycloid and vertebrae completely or nearly completely ossified.
_Oligopleurus_, from the Jurassic of England and France; _Oenoscopus_, from
the Jurassic and Cretaceous of France, Germany, and Italy; and
_Spathiurus_, from the Cretaceous of Mount Lebanon and Dalmatia.
{546}FAM. 4. LEPTOLEPIDIDAE.—Praemaxillaries very small; maxillaries large,
loosely attached; teeth small and conical. Parietal bones separating the
supraoccipital from the frontals; opercular bones well developed. Vertebral
centra well ossified, but always pierced by the notochord; ribs delicate;
epipleurals present; no fused or expanded haemal arches at the base of the
caudal fin. Dorsal and anal fins small, the former above or behind the
ventrals. Ventrals with 5 to 10 rays. Scales thin, cycloid and deeply
imbricate, usually coated with ganoin in their exposed portion.
[Illustration: FIG. 325.—_Leptolepis dubius_. (Restoration of skeleton by
A. S. Woodward.)]
_Leptolepis_, with numerous species, from the Jurassic and Cretaceous of
Europe and New South Wales; _Vidalia_, Jurassic of France; _Aethalion_,
Jurassic of Bavaria; _Thrissops_, Jurassic and Cretaceous of Europe; and
_Lycoptera_, Jurassic of Asia.
FAM. 5. ELOPIDAE.—Margin of the upper jaw formed by the praemaxillaries and
the maxillaries, the latter the more developed, and movably articulated
above the former to the ethmoid. Parietal bones in contact behind the
frontals; opercular bones well developed. Basis cranii double. A bony
intergular or sublingual plate. Jaws, palatines, pterygoids, vomer,
parasphenoid, glossohyal, and pharyngeals toothed. Ribs mostly sessile,
inserted very low down, behind parapophyses; epineurals similar to the
ribs, but directed upwards. Pectorals low down, folding like the ventrals.
Post-temporal forked, the upper branch attached to the epiotic, the lower
to the opisthotic; post-clavicle small; scapular foramen in the scapula;
pterygials well developed, three in contact with coracoid. Ventrals with 10
to 16 rays. Branchiostegal rays very numerous (over 20). Air-bladder large.
{547}This family is abundantly represented in Cretaceous times by the
genera _Osmeroides_ and _Elopopsis_, and from the Lower Eocene to the
present day by _Elops_ and _Megalops_. _Elops saurus_ is a handsome
elongate silvery Fish, found in all the warm and tropical seas; the young
are ribbon-shaped like those of _Albula_. A second species, _E. lacerta_,
is from the West Coast of Africa, entering rivers. _Megalops_,
distinguished by larger scales, the absence of pseudobranchiae, and the
curious prolongation of the last ray of the dorsal fin, includes the
well-known Tarpon _M. atlanticus_, and the Indian _M. cyprinoides_. The
Tarpon occurs from the south-eastern coasts of North America and the West
Indies to Brazil, and reaches a length of 6 feet and a weight of 110 lbs.
It often leaps out of the water, after the manner of Grey Mullets, and its
chase when hooked affords good sport, the landing of so active a giant
being attended with great difficulties. Its remarkably large scales, over
two inches in diameter, are much prized for fancy work in the Florida
curiosity shops.
[Illustration: FIG. 326.—Tarpon, _Megalops atlanticus_, much reduced.
(After Goode.)]
FAM. 6. ALBULIDAE.—Margin of the upper jaw formed by the praemaxillaries
and the maxillaries, the latter movably articulated above the former to the
ethmoid. Parietal bones separating the supraoccipital from the frontals;
suboperculum large; interoperculum small and entirely or nearly entirely
hidden below the praeoperculum. Basis cranii double. Praemaxillaries,
mandible, vomer, palatines, pterygoid, parasphenoid, glossohyal, and
pharyngeals toothed. Ribs sessile, inserted behind and somewhat below small
parapophyses, which are absent or merely indicated on the anterior
vertebrae, and gradually increase in size towards the caudal region; these
parapophyses, as well as the neural and haemal arches, are autogenous
bones; epineurals, no epipleurals. {548}Pectorals low down, folding like
the ventrals. Post-temporal trifid, the upper branch attached to the
epiotic, the median to the squamosal, the lower to the opisthotic;
post-clavicle large (formed of three pieces); scapular foramen between
scapula and clavicle; pterygials well developed, two in contact with
coracoid. Ventrals with 10 to 14 rays. Branchiostegal rays 6 to 14.
Air-bladder large, not communicating with the ear.
Elongate fusiform Fishes, covered with large silvery scales forming regular
longitudinal series; head naked; mouth rather small, with thick lips;
gill-openings wide. Dorsal fin originating in front of ventrals; anal
short; caudal well developed, forked.
[Illustration: FIG. 327.—_Albula conorhynchus_. ¼ nat. size.]