CHAPTER XXXI

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."

=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.

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.