THE CLASS ELASMOBRANCHII OR SHARK-LIKE FISHES
=The Sharks.=--The gap between the lancelets and the lampreys is a
very wide one. Assuming the primitive nature of both groups, this
gap must represent the period necessary for the evolution of brain,
skull, and elaborate sense organs. The interspace between the lampreys
and the nearest fish-like forms which follow them in an ascending
scale is not less remarkable. Between the lamprey and the shark we
have the development of paired fins with their basal attachments
of shoulder-girdle and pelvis, the formation of a lower jaw, the
relegation of the teeth to the borders of the mouth, the development
of separate vertebræ along the line of the notochord, the development
of the gill-arches, and of an external covering of enameled points or
placoid scales.
These traits of progress separate the Elasmobranchs from all lower
vertebrates. For those animals which possess them, the class name of
_Pisces_ or fishes has been adopted by numerous authors. If this term
is to be retained for technical purposes, it should be applied to the
aquatic vertebrates above the lampreys and lancelets. We may, however,
regard fish as a popular term only, rather than to restrict the name to
members of a class called _Pisces_. From the bony fishes, on the other
hand, the sharks are distinguished by the much less specialization
of the skeleton, both as regards form and substance, by the lack of
membrane bones, of air-bladder, and of true scales, and by various
peculiarities of the skeleton itself. The upper jaw, for example, is
formed not of maxillary and premaxillary, but of elements which in
the lower fishes would be regarded as belonging to the palatine and
pterygoid series. The lower jaw is formed not of several pieces,
but of a cartilage called Meckel's cartilage, which in higher fishes
precedes the development of a separate dentary bone. These structures
are sometimes called primary jaws, as distinguished from secondary jaws
or true jaws developed in addition to those bones in the _Actinopteri_
or typical fishes. In the sharks the shoulder-girdle is attached, not
to the skull, but to a vertebra at some distance behind it, leaving
a distinct neck, such as is possessed or retained by the vertebrate
higher than fishes. The shoulder-girdle itself is a continuous arch of
cartilage, joining its fellow at the breast of the fish. Other peculiar
traits will be mentioned later.
=Characters of Elasmobranchs.=--The essential character of the
Elasmobranchs as a whole are these: The skeleton is cartilaginous, the
skull without sutures, and the notochord more or less fully replaced
or inclosed by vertebral segments. The jaws are peculiar in structure,
as are also the teeth, which are usually highly specialized and found
on the jaws only. There are no membrane bones; the shoulder-girdle is
well developed, each half of one piece of cartilage, and the ventral
fins, with the pelvic-girdle, are always present, always many-rayed,
and abdominal in position. The skin is covered with placoid scales,
or shagreen, or with bony bucklers, or else it is naked. It is never
provided with imbricated scales. The tail is diphycercal, heterocercal,
or else it degenerates into a whip-like organ, a form which has been
called leptocercal. The gill-arches are 5, 6, or 7 in number, with
often an accessory gill-slit or spiracle. The ventral fins in the
males (except perhaps in certain primitive forms) are provided with
elaborate cartilaginous appendages or claspers. The brain is elongate,
its parts well separated, the optic nerves interlacing. The heart has
a contractile arterial cone containing several rows of valves; the
intestine has a spiral valve; the eggs are large, hatched within the
body, or else deposited in a leathery case.
=Classification of Elasmobranchs.=--The group of sharks and their
allies, rays, and Chimæras, is usually known collectively as
_Elasmobranchii_ (~elasmos~, blade or plate; ~branchos~, gill). Other
names applied to all or a part of this group are these: _Selachii_
(~selachos~, a cartilage, the name also used by the Greeks for the
gristle-fishes or sharks); _Plagiostomi_ (~plagios~, oblique; ~stoma~,
mouth); _Chondropterygii_ (~chondros~, cartilage; ~pteryx~, fin); and
_Antacea_ (~antakaios~, sturgeon). They represent the most primitive
known type of jaw-bearing vertebrates, or _Gnathostomi_ (~gnathos~,
jaw; ~stoma~, mouth), the Chordates without jaws being sometimes called
collectively _Agnatha_ (~a-gnathos~, without jaws). These higher types
of fishes have been also called collectively _Lyrifera_, the form
of the two shoulder-girdles taken together being compared to that
of a lyre. Through shark-like forms all the higher vertebrates must
probably trace their descent. Sharks' teeth and fin-spines are found
in all rocks from the Upper Silurian deposits to the present time, and
while the majority of the genera are now extinct, the class has had a
vigorous representation in all the seas, later Palæozoic, Mesozoic, and
Cenozoic, as well as in recent times.
Most of the Elasmobranchs are large, coarse-fleshed, active animals
feeding on fishes, hunting down their prey through superior strength
and activity. But to this there are many exceptions, and the highly
specialized modern shark of the type of the mackerel-shark or man-eater
is by no means a fair type of the whole great class, some of the
earliest types being diminutive, feeble, and toothless.
=Subclasses of Elasmobranchs.=--With the very earliest recognizable
remains it is clear that the Elasmobranchs are already divided into two
great divisions, the sharks and the _Chimæras_. These groups we may
call subclasses, the _Selachii_ and the _Holocephali_, or Chismopnea.
The _Selachii_, or sharks and rays, have the skull hyostylic, that is,
with the quadrate bone grown fast to the palate which forms the upper
jaw, the hyomandibular, acting as suspensorium to the lower jaw, being
articulated directly to it.
The palato-quadrate apparatus, the front of which forms the upper jaw
in the shark, is not fused to the cranium, although it is sometimes
articulated with it. There are as many external gill-slits as there
are gill-arches (5, 6, or 7), and the gills are adnate to the flesh of
their own arches, without free tips. The cerebral hemispheres are grown
together. The teeth are separated and usually strongly specialized,
being primitively modified from the prickles or other defences of the
skin. There is no frontal holder or bony hook on the forehead of the
male.
The subclass _Holocephali_, or _Chimæras_, differ from the sharks in
all this series of characters, and its separation as a distinct group
goes back to the Devonian or even farther, the earliest known sharks
having little more in common with Chimæras than the modern forms have.
=The Selachii.=--There have been many efforts to divide the sharks and
rays into natural orders. Most writers have contented themselves with
placing the sharks in one order (_Squali_ or _Galei_ or _Pleurotremi_)
having the gill-openings on the side, and the rays in another (_Rajæ_,
_Batoidei_, _Hypotrema_) having the gill-openings underneath. Of far
more importance than this superficial character of adaptation are the
distinctions drawn from the skeleton. Dr. Gill has used the attachment
of the palato-quadrate apparatus as the basis of a classification. The
_Opistharthri_ (_Hexanchidæ_) have this structure articulated with the
postorbital part of the skull. In the _Prosarthri_ (_Heterodontidæ_)
it is articulated with the preorbital part of the skull, while in the
other sharks (_Anarthri_) it is not articulated at all. But these
characters do not appear to be always important. _Chlamydoselachus_,
for example, differs in this regard from _Heptranchias_, which in
other respects it closely resembles. Yet, in general, the groups thus
characterized are undoubtedly natural ones.
[Illustration: FIG. 298.--Fin-spine of _Onchus tenuistriatus_ Agassiz.
(After Zittel.)]
=Hasse's Classification of Elasmobranchs.=--In 1882, Professor Carl
Hasse proposed to subdivide the sharks on the basis of the structure
of the individual vertebræ. In the lowest group, a hypothetical
order of _Polyospondyli_, possibly represented by the fossil spines
called _Onchus_, an undivided notochord, perhaps swollen at regular
intervals, is assumed to have represented the vertebral column.
In the _Diplospondyli_ (_Hexanchidæ_) the imperfectly segmented
vertebræ are joined in pairs, each pair having two neural arches.
In the _Asterospondyli_ or ordinary sharks each vertebra has its
calcareous lamella radiating star-like from the central axis. In the
_Cyclospondyli_ (_Squalidæ_, etc.) the calcareous part forms a single
ring about the axis, and in the _Tectospondyli_ (_Squatina_, rays,
etc.) it forms several rings. These groups again are natural and
correspond fairly with those based on other characters. At the same
time there is no far-reaching difference between _Cyclospondyli_ and
_Tectospondyli_, and the last-named section includes both sharks and
rays.
[Illustration: FIG. 299.--Section of vertebræ of sharks, showing
calcification. (After Hasse.) 1. _Cyclospondyli_ (_Squalus_); 2.
_Tectospondyli_ (_Squatina_); 3. _Asterospondyli_ (_Carcharias_).]
Nothing is known of the _Polyospondyli_, and they may never have
existed at all. The _Diplospondyli_ do not differ very widely from the
earlier _Asterospondyli_ (_Cestraciontes_) which, as a matter of fact,
have preceded the _Diplospondyli_ in point of time, if we can trust our
present knowledge of the geological record.
=Other Classifications of Elasmobranchs.=--Characters more fundamental
may be drawn from the structure of the pectoral fin. In this regard
four distinct types appear. In _Acanthoessus_ this fin consists of
a stout, stiff spine, with a rayless membrane attached behind it.
In _Cladoselache_ the fin is low, with a very long base, like a
fold of skin (_ptychopterygium_), and composed of feeble rays. In
_Pleuracanthus_ it is a jointed axis of many segments, with a fringe
of slender fin-rays, corresponding in structure to all appearance to
the pectoral fin of Dipnoans and Crossopterygians, the type called by
Gegenbaur _archipterygium_ on the hypothesis that it represents the
primitive vertebrate limb.
In most sharks the fin has a fan-shape, with three of the basal
segments larger than the others. Of these the mesopterygium is the
central one, with the propterygium before it and the metapterygium
behind. In the living sharks of the family of _Heterodontidæ_, this
form of fin occurs and the teeth of the same general type constitute
the earliest remains distinctly referable to sharks in the Devonian
rocks.
=Primitive Sharks.=--Admitting that these four types of pectoral fin
should constitute separate orders, we have next to consider which form
is the most primitive and what is the line of descent. In this matter
we have, in the phrase of Hæckel, only the "three ancestral documents,
Palæontology, Morphology, and Ontogeny."
Unfortunately the evidence of these documents is incomplete and
conflicting. So far as Palæontology is concerned, the fin of
_Cladoselache_, with that of _Acanthoessus_, which may be derived
from it, appears earliest, but the modern type of pectoral fin with
the three basal segments is assumed to have accompanied the teeth of
Psammodonts and Cochliodonts, while the fin of the Chimæra must have
been developed in the Devonian. The jointed fin of _Cladodus_ and
_Pleuracanthus_ may be a modification or degradation of the ordinary
type of shark-fin.
Assuming, however, that the geological record is not perfect and that
the fin of _Cladoselache_ is not clearly shown to be primitive, we have
next to consider the evidence drawn from morphology.
Those who with Balfour and others (see page 69) accept the theory
that the paired fins are derived from a vertebral fold, will regard
with Dean the fin of _Cladoselache_ as coming nearest the theoretical
primitive condition.
The pectoral fin in _Acanthoessus_ Dean regards as a specialized
derivative from a fin like that of _Cladoselache_, the fin-rays being
gathered together at the front and joined together to form the thick
spine characteristic of _Acanthoessus_. This view of the morphology of
the fin of _Acanthoessus_ is not accepted by Woodward, and several
different suggestions have been recorded.
If with Gegenbaur we regard the paired fins as derived from the septa
between the gill-slits, or with Kerr regard them as modified external
gills, the whole theoretical relation of the parts is changed. The
archipterygium of _Pleuracanthus_ would be the nearest approach to
the primitive pectoral limb, and from this group and its allies
all the other sharks are descended. This central jointed axis of
_Pleuracanthus_ is regarded by Traquair as the equivalent of the
metapterygium in ordinary sharks. (See Figs. 44, 45, 46.)
According to Traquair: "The median stern [of the archipterygium],
simplified, shortened up and losing all its radials on the
postaxial side, except in sometimes a few near the tip, becomes the
metapterygium, while the mesopterygium and propterygium are formed by
the fusion into two pieces of the basal joints of a number of preaxial
radials, which have reached and become attached to the shoulder-girdle
in front of the metapterygium."
According to Dr. Traquair, the pectoral fin in _Cladodus neilsoni_, a
shark from the Coal Measures of Scotland, is "apparently a veritable
uniserial archipterygium midway between the truly biserial one of
_Pleuracanthus_ and the pectoral fin of ordinary sharks." Other
authors look on these matters differently, and Dr. Traquair admits
that an opposite view is almost equally probable. Cope and Dean
would derive the tribasal pectoral of ordinary sharks directly
from the ptychopterygium or fan-like fold of _Cladoselache_, while
Fritsch and Woodward would look upon it as derived in turn from the
_Ceratodus_-like fin of _Pleuracanthus_, itself derived from the
ptychopterygium or remains of a lateral fin-fold.
If the Dipnoans are descended from the Crossopterygians, as Dollo
has tried to show, the archipterygium of _Pleuracanthus_ has had a
different origin from the similar-appearing limb of the Dipnoans,
_Dipterus_ and _Ceratodus_.
In such case the archipterygium would not be the primitive pectoral
limb, but a structure which may have been independently evolved in two
different groups.
In the view of Gegenbaur, the Crossopterygians and Dipnoans with all
the higher vertebrates and the bony fishes would arise from the same
primitive stock, ancestors, or allies of the _Ichthyotomi_, which group
would also furnish the ancestors of the _Chimæras_. In support of this
view, the primitive protocercal or diphycercal tail of _Pleuracanthus_
may be brought in evidence as against the apparently more specialized
heterocercal tail of _Cladoselache_. But this is not conclusive, as
the diphycercal tail may arise separately in different groups through
degeneration, as Dollo and Boulenger have shown.
The matter is one mainly of morphological interpretation, and no final
answer can be given. On page 68 a summary of the various arguments
may be found. Little light is given by embryology. The evidence of
Palæontology, so far as it goes, certainly favors the view of Balfour.
Omitting detached fin-spines and fragments of uncertain character, the
earliest identifiable remains of sharks belong to the lower Devonian.
These are allies of _Acanthoessus_. _Cladoselache_ comes next in the
Upper Devonian. _Pleuracanthus_ appears with the teeth and spines
supposed to belong to Cestraciont sharks, in the Carboniferous Age.
The primitive-looking _Notidani_ do not appear before the Triassic.
For this reason the decision as to which is the most primitive type of
shark must therefore rest unsettled for the present and perhaps for a
long time to come.
The weight of authority at present seems to favor the view of Balfour,
Wiedersheim, Boulenger, and Dean, that the pectoral limb has arisen
from a lateral fold of skin. But weight of authority is not sufficient
when evidence is confessedly lacking.
For our purpose, without taking sides in this controversy, we may
follow Dean in allowing _Cladoselache_ to stand as the most primitive
of known sharks, thus arranging the Elasmobranchs and rays, recent and
fossil, in six orders of unequal value--_Pleuropterygii_, _Acanthodei_,
_Ichthyotomi_, _Notidani_, _Asterospondyli_, and _Tectospondyli_. Of
these orders the first and second are closely related, as are also
the fourth and fifth, the sixth being not far remote. The true sharks
form the culmination of one series, the rays of another, while from
the _Ichthyotomi_ the Crossopterygians and their descendants may be
descended. But this again is very hypothetical, or perhaps impossible;
while, on the other hand, the relation of the Chimæras to the sharks
is still far from clearly understood.
=Order Pleuropterygii.=--The order of _Pleuropterygii_ of Dean
(~pleuron~, side; ~pteryx~, fin), called by Parker and Haswell
_Cladoselachea_, consists of sharks in which the pectoral and ventral
fins have each a very wide horizontal base (ptychopterygium), without
jointed axis and without spine. There are no spines in any of the
fins. The dorsal fin is low, and there were probably two of them.
The notochord is persistent, without intercalary cartilage, such as
appear in the higher sharks. The caudal fin is short, broad, and
strongly heterocercal. Apparently the ventral fin is without claspers.
The gill-openings were probably covered by a dermal fold. The teeth
are weak, being modified denticles from the asperities of the skin.
The lateral line is represented by an open groove. The family of
_Cladoselachidæ_ consists of a single genus _Cladoselache_ from the
Cleveland shale or Middle Devonian of Ohio. _Cladoselache fyleri_ is
the best-known species, reaching a length of about two feet. Dean
regards this as the most primitive of the sharks, and the position of
the pectorals and ventrals certainly lend weight to Balfour's theory
that they were originally derived from a lateral fold of skin. I am
recently informed by Dr. Dean that he has considerable evidence that
in _Cladoselache_ the anus was _subterminal_. If this statement is
verified, it would go far to establish the primitive character of
_Cladoselache_.
[Illustration: FIG. 300.--_Cladoselache fyleri_ (Newberry), restored.
Upper Devonian of Ohio. (After Dean.)]
=Order Acanthodei.=--Near the _Pleuropterygii_, although much more
highly developed, we may note the strange group of _Acanthodei_
(~akanthôdês~, spinous). These armed fishes were once placed among
the Crossopterygians, but there seems no doubt that Woodward is right
in regarding them as a highly specialized aberrant offshoot of the
primitive sharks. In this group the paired fins consist each of a
single stout spine, nearly or quite destitute of other rays. A similar
spine is placed in front of the dorsal fin and one in front of the
anal. According to Dean these spines are each produced by the growing
together of all the fin-rays normally belonging to the fin, a view of
their morphology not universally accepted.
[Illustration: FIG. 301.--_Cladoselache fyleri_ (Newberry), restored.
Ventral view. (After Dean.)]
[Illustration: FIG. 302.--Teeth of _Cladoselache fyleri_ (Newberry).
(After Dean.)]
[Illustration: FIG. 303.--_Acanthoessus wardi_ (Egerton).
Carboniferous. Family _Acanthoessidæ_. (After Woodward.)]
The dermal covering is highly specialized, the shagreen denticles being
much enlarged and thickened, often set in little squares suggesting a
checker-board. The skull is covered with small bony plates and membrane
bones form a sort of ring about the eye. The teeth are few, large, and
"degenerate in their fibrous structure." Some of the species have
certainly no teeth at all. The tail is always heterocercal, or bent
upward at tip as in the _Cladoselache_, not diphycercal, tapering and
horizontal as in the _Ichthyotomi_.
The lower Acanthodeans, according to Woodward, "are the only
vertebrates in which there are any structures in the adult apart
from the two pairs of fins which may be plausibly interpreted as
remnants of once continuous lateral folds. In _Climatius_, one of
the most primitive genera (see Fig. 305), there exists, according
to Woodward, and as first noticed by Cope, between the pectoral and
pelvic (or ventral) fins a close and regular series of paired spines,
in every respect identical with those supporting the appendages
that presumably correspond to the two pairs of fins in the higher
genera. They may even have supported fin membranes, though specimens
sufficiently well preserved to determine this point have not yet been
discovered. However, it is evident that dermal calcifications attained
a greater development in the _Acanthodei_ than in any of the more
typical Elasmobranchs, and we may look for much additional information
on the subject when the great fishes to which the undetermined
_Ichthyodorulites_ pertained became known." (See Fig. 305.)
The _Acanthodei_ constitute three families. In the _Acanthoessidæ_
there is but one short dorsal fin opposite the anal, and clavicular
bones are absent. The gill-openings being provided with "frills" or
collar-like margins, perhaps resembled those of the living genus
_Chlamydoselachus_, the frilled shark. The pectoral spine is very
strong, and about the eye is a ring of four plates. The body is
elongate, tapering, and compressed. _Acanthoessus_ of Agassiz, the name
later changed by its author to _Acanthodes_, is the principal genus,
found in the Devonian and Carboniferous.
The species of _Acanthoessus_ are all small fishes rarely more than a
foot long, with very small teeth or none, and with the skin well armed
with a coat-of-mail. _Acanthoessus bronni_ is the one longest known.
In the earliest species known, from the Devonian, the ventral fins are
almost as large as the pectorals and nearly midway between pectorals
and anal. In the later species the pectoral fins become gradually
larger and the ventrals move forward. In the Permian species the
pectorals are enormous.
_Traquairia pygmæa_, from the Permian of Bohemia, is a diminutive
sharklet three or four inches long with large scales, slender spines,
and apparently no ventral fins.
In the genus _Cheiracanthus_ the dorsal fin is placed before the anal.
In _Acanthodopsis_ the teeth are few, large, and triangular, and the
fin-spines relatively large.
The _Ischnacanthidæ_ have no clavicles, and two dorsal fins.
_Ischnacanthus gracilis_ of the Devonian has a few large conical teeth
with small cusps between them.
The _Diplacanthidæ_, with two dorsal fins, possess bones interpreted as
clavicles. The teeth are minute or absent. In _Diplacanthus striatus_
and _Diplacanthus longispinus_ of the Lower Devonian stout spines are
attached to the shoulder-girdle between the pectoral spines below.
[Illustration: FIG. 304.--_Diplacanthus crassissimus_ Duff. Devonian.
Family _Diplacanthidæ_. (After Nicholson). (Restoration of jaws and
gill-openings; after Traquair.)]
In the very small sharks called _Climatius_ the fin-spines are very
strong, and a series of several free spines occurs, as above stated, on
each side between the pectoral and ventral fins, a supposed trace of a
former lateral fold. In _Paraxus_ the first dorsal spine is enormously
enlarged in size, the other spines remaining much as in _Climatius_.
=Dean on Acanthodei.=--In his latest treatise on these fishes,
"The Devonian Lamprey," Dr. Dean unites the _Pleuropterygii_ and
_Acanthodei_ in a single order under the former name, regarding
_Acanthoessus_ as an ally and perhaps descendant of the primitive
_Cladoselache_. Dr. Dean observes:
"In the foregoing classification it will be noted that the
Acanthodia are regarded as included under the first order of sharks,
_Pleuropterygii_. To this arrangement Smith Woodward has already
objected that the spines of Acanthodians cannot be regarded as the
homologues of the radial elements of the Cladoselachian fin (which by
a process of concrescence have become fused in its interior margin),
since he believes the structure to be entirely dermal in origin. His
criticism, however, does not seem to me to be well grounded, for,
although all will admit that Acanthodian spines have become incrusted,
and deeply incrusted, with a purely dermal calcification, it does
not follow that the interior of the spine has not had primitively
a non-dermal core. That the concrescence of the radial supporting
elements of the fin took place _pari passu_ with the development of a
strengthening dermal support of the fin margin was the view expressly
formulated in my previous paper on this subject. It is of interest in
this connection to recall that the earliest types of Acanthodian spines
were the widest, and those which, in spite of their incasing dermal
calcification, suggest most clearly the parallel elements representing
the component radial supports. There should also be recalled the
many features in which the Acanthodians have been shown to resemble
_Cladoselache_."
[Illustration: FIG. 305.--_Climatius scutiger_ Egerton, restored.
Family _Diplacanthidæ_. (After Powrie, per Zittel.)]
From these primitive extinct types of shark we may proceed to
those forms which have representatives among living fishes. From
_Cladoselache_ a fairly direct series extends through the _Notidani_
and _Cestraciontes_, culminating in the Lamnoid and Galeoid sharks.
Still another series, destitute of anal fin, probably arising near the
_Acanthodei_, reaches its highest development in the side branch of the
_Batoidei_ or rays. The _Holocephali_ and _Dipneusti_ must also find
their origin in some of these primitive types, certainly not in any
form of more highly specialized sharks.
[Illustration: FIG. 306.--_Pleuracanthus decheni_ Goldfuss. Family
_Pleuracanthidæ_. (After Roemer, per Zittel.)]
Woodward prefers to place the _Tectospondyli_ next to the
_Ichthyotomi_, leaving the specialized sharks to be treated later.
There is, however, no linear system which can interpret natural
affinities, and we follow custom in placing the dogfishes and rays at
the end of the shark series.
[Illustration: FIG. 307.--_Pleuracanthus decheni_, restored. (After
Brongniart.) The anterior anal very hypothetical.]
[Illustration: FIG. 308.--Head-bones and teeth of _Pleuracanthus
decheni_ Goldfuss. (After Davis, per Dean.)]
[Illustration: FIG. 309.--Teeth of _Didymodus bohemicus_ Quenstadt.
Carboniferous. Family _Pleuracanthidæ_. (After Zittel)]
=Order Ichthyotomi.=--In the order _Ichthyotomi_ (~ichthys~, fish;
~tomos~, cutting; named by Cope from the supposed segmentation of
the cranium; called by Parker and Haswell _Pleuracanthea_) the very
large pectoral fins are developed each as an archipterygium. Each fin
consists of a long segmented axis fringed on one or both sides with
fin-rays. The notochord is very simple, scarcely or never constricted,
the calcifications of its sheath "arrested at the most primitive or
rhachitomous stage, except in the tail." This is the best defined of
the orders of sharks, and should perhaps rank rather as a subclass,
as the _Holocephali_. Two families of _Ichthyotomi_ are recognized
by Woodward, the _Pleuracanthidæ_ and the _Cladodontidæ_. In the
_Pleuracanthidæ_ the dorsal fin is long and low, continuous from head
to tail, and the pectoral rays are in two rows. There is a long barbed
spine with two rows of serrations at the nape. The body is slender,
not depressed, and probably covered with smooth skin. The teeth have
two or more blunt cusps, sometimes with a smaller one between and a
blunt button behind. The interneural cartilages are more numerous than
the neural spines. The genera are imperfectly known, the skeleton of
_Pleuracanthus decheni_ only being well preserved. This is the type
of the genus called _Xenacanthus_ which, according to Woodward, is
identical with _Pleuracanthus_, a genus otherwise known from spines
only. The denticles on the spine are straight or hooked backward,
in _Pleuracanthus_ (_lævissimus_), the spine being flattened. In
_Orthacanthus_ (_cylindricus_), the spine is cylindrical in section.
The species called _Dittodus_ and _Didymodus_ are known from the
teeth only. These resemble the teeth of _Chlamydoselachus_. It is not
known that _Dittodus_ possesses the nuchal spine, although detached
spines like those of _Pleuracanthus_ lie about in remains called
_Didymodus_ in the Permian rocks of Texas. In _Dicranodus texensis_
the palato-quadrate articulates with the postorbital process of the
cranium, as in the _Hexanchidæ_, and the hyomandibular is slender.
[Illustration: FIG. 310.--Shoulder-girdle and pectoral fins of
_Cladodus neilsoni_ Traquair.]
A genus, _Chondrenchelys_, from the sub-Carboniferous of Scotland, is
supposed to belong to the _Pleuracanthidæ_, from the resemblance of
the skeleton. It has no nuchal spine, and no trace of paired fins is
preserved.
The _Cladodontidæ_ differ in having the "pectoral fin developed in
the form of a uniserial archipterygium intermediate between the truly
biserial one of _Pleuracanthus_ and the pectoral fin of modern sharks."
The numerous species are known mainly from detached teeth, especially
abundant in America, the earliest being in the Lower Carboniferous. One
species, _Cladodus nelsoni_ (Fig. 310), described by Traquair, from
the sub-Carboniferous of Scotland shows fairly the structure of the
pectoral fin.
[Illustration: FIG. 311.--Teeth of _Cladodus striatus_ Agassiz. (After
Davis.) Carboniferous.]
In _Cladodus mirabilis_ the teeth are very robust, the crown consisting
of a median principal cone and two or three large lateral cones on each
side. The cones are fairly striate. In _Lambdodus_ from Illinois there
are no lateral cones. Other genera are _Dicentrodus_, _Phoebodus_,
_Carcharopsis_, and _Hybocladodus_.