THE HOLOCEPHALI, OR CHIMÆRAS
=The Chimæras.=--Very early in geological times, certainly as early as
the middle Silurian, the type of _Chimæras_ diverged from that of the
sharks. Hasse derives them directly from his hypothetical primitive
_Polyospondyli_, by way of the _Acanthodei_ and _Ichthyotomi_. In
any event the point of divergence must be placed very early in the
evolution of sharks, and this suggestion is as likely as any other.
The chief character of Chimæras is found in the autostylic skull,
which is quite different from the hyostylic skull of the sharks. In
the sharks and in all higher fishes the mandible is joined to the
skull by a suspensorium of bones or cartilages (quadrate, symplectic,
and hyomandibular bones in the Teleost fishes). To this arrangement
the name hyostylic is given. In the Chimæra there is no suspensorium,
the mandible being directly attached to the cranium, of which the
hyomandibular and quadrate elements form an integral part, this
arrangement being called autostylic. The palato-quadrate apparatus, of
which the upper jaw is the anterior part, is immovably fused with the
cranium, instead of being articulated with it. This fact, gives the
name to the subclass _Holocephali_ (~holos~, whole or solid; ~kephalê~,
head). Other characters are found in the incomplete character of the
back-bone, which consists of a scarcely segmented notochord differing
from the most primitive condition imagined only in being surrounded
by calcareous rings, no lime entering into the composition of the
notochord itself. The tail is diphycercal and usually prolonged in
a filament (leptocercal). The shoulder-girdle, as in the sharks, is
free from the skull. The pectoral fins are short and broad, without
segmented axis or archipterygium and without recognizable analogue
of the three large cartilages seen in the sharks, the propterygium,
mesopterygium, and metapterygium. In the mouth, instead of teeth, are
developed flat, bony plates called tritors or grinders, set endwise in
the front of the jaws. The gills are fringe-like, free at the tips as
in ordinary fishes, and there is a single external opening for them
all as in true fishes, and they are covered with a flap of skin. These
structures are, however, quite different from those of the true fishes
and are doubtless independently developed. There is no spiracle. The
skin is smooth or rough. In the living forms and most of the extinct
species there is a strong spine in the dorsal fin. The ventral fin
in the male has complex, usually trifid, claspers, and an analogous
organ, the cephalic holder, is developed on the front of the head,
in the adult male. This is a bony hook with a brush of glistening
enameled teeth at the end. The eggs are large, and laid in oblong or
elliptical egg-cases, provided with silky filaments. The eggs are
fertilized after they are extruded. Mucous channels and lateral line
are highly developed, being most complex about the head. The brain
is essentially shark-like, the optic nerves form a chiasma, and the
central hemispheres are large.
The teeth of the Chimæras are thus described by Woodward, vol. 2, pp.
36, 37:
"In all the known families of Chimæroids, the dentition consists
of a few large plates of vascular dentine, of which certain areas
('tritors') are specially hardened by the deposition of calcareous
salts within and around groups of medullary canals, which rise at right
angles to the functional surface. In most cases there is a single pair
of such plates in the lower jaw, meeting at the symphysis, while two
pairs are arranged to oppose these above. As a whole, the dentition
thus closely resembles that of the typical Dipnoi (as has often been
pointed out); and the upper teeth may be provisionally named palatine
and vomerine until further discoveries shall have revealed their
precise homologies. The structures are sometimes described as 'jaws,'
and regarded as dentaries, maxillæ, and premaxillæ, but the presence
of a permanent pulp under each tooth is conclusive proof of their
bearing no relation to the familiar membrane-bones thus named in higher
fishes."
=Relationship of Chimæras.=--As to the origin of the Chimæras and
their relation to the sharks, Dr. Dean has this recent ("The Devonian
Lamprey") and interesting word:
"The Holocephali have always been a doubtful group, anatomy and
palæontology contributing but imperfect evidence as to their
position in the gnathostome phylum. Their embryology, however, is
still undescribed, except in a brief note by T. J. Parker, and it
is reasonably looked to to contribute evidence as to their line of
descent. The problem of the relationships of the Chimæroids has long
been of especial interest to me, and it has led me to obtain embryonic
material of a Pacific species of one of these forms. It may be of
interest in this connection to state that the embryology of this form
gives the clearest evidence that the wide separation of the Selachii
and Holocephali is not tenable. The entire plan of development in
_Chimæra colliei_ is clearly like that of a shark. The ovulation
is closely like that of certain of the rays and sharks: the eggs
are large, the segmentation is distinctly shark-like; the circular
blastoderm overgrows the yolk in an elasmobranchian manner. The early
embryos are shark-like; and the later ones have, as T. J. Parker has
shown, external gills, and I note further that these arise, precisely
as in shark-embryos, from the posterior margin of the gill-bar. A
spiracle also is present. A further and most interesting developmental
feature is the fact that the autostylism in _Chimæra_ is purely of
secondary nature and is at the most of ordinal value. It is found that
in a larva of _Chimæra_ measuring 45 mm. in length, the palato-quadrate
cartilage is still separated from the skull by a wide fissure. This
becomes gradually reduced by the confluence of the palato-quadrate
cartilage with the skull, the fusion taking place at both the anterior
and posterior ends of the mesal rim of the cartilage. The remains of
the fissure are still well marked in the young _Chimæra_, four inches
in length; and a rudiment of it is present in the adult skull as a
passage-way for a nerve. Regarding the dentition: it may also be noted
in the present connection that the growth of the dental plates in
_Chimæra_ suggests distinctly elasmobranchian conditions. Thus on the
roof of the mouth the palatine plates are early represented by a series
of small more or less conical elements which resemble outwardly, at
least, the 'anlagen' of the pavement teeth in cestraciont sharks."
[Illustration: FIG. 351.--Skeleton of _Chimæra monstrosa_ Linnæus.
(After Dean.)]
=Family Chimæridæ.=--The existing Chimæras are known also as
spookfishes, ratfishes, and elephant-fishes. These are divided
by Garman into three families, and in the principal family, the
_Chimæridæ_, the snout is blunt, the skin without plates, and the
dorsal fin is provided with a long spine. The flat tritors vary in the
different genera. The single genus represented among living fishes
is _Chimæra_, found in cold seas and in the oceanic depths. The
best-known species, _Chimæra colliei_, the elephant-fish, or chimæra
of California, abounds in shallow waters of ten to twenty fathoms from
Sitka to San Diego. It is a harmless fish, useless except for the oil
in its liver, and of special interest to anatomists as the only member
of the family to be found when desired for dissection. This species
was first found at Monterey by Mr. Collie, naturalist of Captain
Beechey's ship, the _Blossom_. It is brown in color, with whitish
spots, and reaches a length of 2-1/2 feet. As a shallow-water form,
with certain differences in the claspers and in the tail, _Chimæra
colliei_ is sometimes placed in a distinct genus, _Hydrolagus_. Other
species inhabit much greater depths and have the tail produced into
a long filament. Of these, _Chimæra monstrosa_, the sea-cat of the
north Atlantic, has been longer known than any other Chimæra. _Chimæra
affinis_ has been dredged in the Gulf Stream and off Portugal. _Chimæra
phantasma_ and _Chimæra mitsukurii_ are frequently taken in Japan, and
the huge jet-black _Chimæra purpurascens_ in Hawaii and Japan. None of
these species are valued as food, but all impress the spectator with
their curious forms.
[Illustration: FIG. 352.--Elephant-fish, _Chimæra colliei_ Lay &
Bennett. Monterey.]
The fossil _Chimæridæ_, although numerous from Triassic times and
referred to several genera, are known chiefly by their teeth with
occasional fin-spines, frontal holders, or impressions of parts of
the skeleton. The earliest of _chimæroid_ remains has been described
by Dr. Charles D. Walcott[151] from Ordovician or Lower Silurian
rocks at Cañon City, Colorado. Of the species called _Dictyorhabdus
priscus_, only parts supposed to be the sheath of the notochord have
been preserved. Dr. Dean thinks this more likely to be part of the axis
of a cephalopod shell. The definitely known _Chimæridæ_ are mainly
confined to the rocks of the Mesozoic and subsequent eras. _Ischyodus
priscus_ (_avitus_) of the lower Jura resembles a modern chimæra.
_Granodus oweni_ is another extinct chimæra, and numerous fin-spines,
teeth, and other fragments in the Cretaceous and Eocene of America
and Europe are referred to _Edaphodon_. A species of _Chimæra_ has
been recorded from the Pliocene of Tuscany, and one of _Callorhynchus_
from the greensand of New Zealand. Other American Cretaceous genera of
chimæroids are _Mylognathus_, _Bryactinus_, _Isotænia_, _Leptomylus_,
and _Sphagepoea_. Dental plates called _Rhynchodus_ are found in the
Devonian.
=Rhinochimæridæ.=--The most degenerate of existing chimæras belong
to the family of _Rhinochimæridæ_, characterized by the long flat
soft blade in which the snout terminates. This structure resembles
that seen in the deep-sea shark, _Mitsukurina_, and in _Polyodon_.
In _Rhinochimæra pacifica_ of Japan the teeth in each jaw form but
a single plate. In _Harriotta raleighana_, of the Gulf Stream, they
are more nearly as in _Chimæra_. Both are bathybial fishes, soft in
texture, and found in great depths. The family of _Callorhynchidæ_, or
Antarctic Chimæras, includes the bottle-nosed Chimæra (_Callorhynchus
callorhynchus_) of the Patagonian region. In this species the snout is
also produced, a portion being turned backward below in front of the
mouth, forming a sensory pad well supplied with nerves.
=Extinct Chimæroids.=--According to Woodward, three other families are
recognizable among the extinct forms.
The _Ptyctodontidæ_ are known from the teeth only, a single pair of
large, laterally compressed dental plates in each jaw, with a few hard
tritoral areas. These occur in Silurian and Devonian rocks. _Ptyctodus
obliquus_ from the Devonian of Russia is the best-known species. Other
genera are _Rhynchodus_ and _Palæomylus_.
The _Squalorajidæ_ have the head depressed and the snout produced in
a flat rostrum, as in _Harriotta_. There is no dorsal spine, and the
teeth are a few thin curved plates. The frontal holder of the male
is well developed. The few species occur in the Lias. _Squaloraja
dolichognathos_ is known from numerous fragments from the Triassic in
England and Scotland. _Chalcodus permianus_ is found in German Permian.
The _Myriacanthidæ_ have the body elongate, with dermal plates on the
head and a long straight spine in the dorsal fin. The frontal holder
is large. The species, few in number, are found in Mesozoic rocks.
_Myriacanthus paradoxus_ is the best-known species. Of another species,
_Chimæropsis paradoxa_, a skeleton about three feet long has been found
which shows a number of peculiar traits. The skin is covered with
ribbed shagreen scales. The dorsal fin has a large spine with retrorse
serrations behind. The tail is slim, and the pectoral and ventral fins
are very large. Bony plates with conical spines protect the neck. The
teeth are large and angular, of peculiar form.
=Ichthyodorulites.=--The term ichthyodorulite (~ichthys~, fish; ~dory~,
lance; ~lithos~, stone) is applied to detached fin-spines, dermal
spines, and tubercles belonging to unrecognized species of sharks and
chimæras. Some of these are serrated, others entire, some straight,
some curved, and some with elaborate armature or sculpture. Some
doubtless belong to _Cestraciontes_, others to _Pleuracanthidæ_; some
to _Squalidæ_, some to chimæras, and others, perhaps, to forms still
altogether unknown.
FOOTNOTES:
[151] Bulletin Geol. Soc. America, 1892.