PLAGIOBTOMI.
(Sharka, Rap.) Char.—Endo-skeleton cartilaginous or partially ossified ; exo skeleton placoid ; gills fixed with five or more gill apertures ; no swim-bladder ; scapular arch detached from the head ; ventrals abdominal ; intestine with spiral valve.* The earliest good evidence which has been obtained of a vertebrate animal in the earth's crust is a spine, of the nature of the dorsal spine of the dog-fish (Acanthias), and a buckler like that of Cephalaspis. Both have been found in the most recent deposits of the Silurian period, in the formation called " upper Ludlow rock." The discovery of the first is due to Murchison ;• its determination to Agassiz, who assigns it to a genus of plagiostomous cartilaginous fishes called Onchus. The buckler was discovered by Mr. Banks, in the " passage beds " of Kington, Herefordshire, and is referred to the genus Pteraspis, Knerr, with microscopic evidence of its piscine nature, by Huxley.
The Onchus spines from the upper Ludlow bone-beds are compressed, slightly curved, less than two inches in length, with no trace at their base of the joint characteristic of the dorsal spines of the " sheat-fishes " (Ganoids of the family Siluridce), or " file-fishes " (Balistidce). The sides of the spine are finely grooved lengthwise, with rounded ribs between the grooves. They are referred to two species—Onchus Murchisoni, and 0. srmistriatus. Sir P. Egerton has lately figured another species from the argillaceous beds near Ludlow, which is more curved, and is armed along the posterior edge ; the longitudinal ribs are fine and numerous, but are constricted at intervals, as in the genus Ctenacanthus, and become subtuberculate at the base. He deems them significant of a distinct genus of shark like fishes.t With the dorsal spines of Onchus are found petrified por tions of skin, tubercular and prickly, like the shagreen of shark's skin, and referred to a genus called Sphagodus ; also coprolitic bodies of phosphate and carbonate of lime, including recognisable parts of the small Mollusks and Crinoids which inhabited the sea-bottom in company with the Onchus-fish. No vertebrae, or other parts of the endo-skeleton of a fish, have been discovered, unless the fragments of a calcified bar, with tooth-like processes, called Plectrodus, be truly jaws with teeth. They resemble, however, parts of the pincer claws of
Crustaceans, as well as of the jaws and teeth of fishes, and do not indicate that class so satisfactorily as the Onchus spines and Sphagodus shagreen. Yet the denticles are confluent with an outer ridge of the bone, according to the " pleurodont" type, and consist of separated large teeth, with minute serial teeth in the interspaces ; and the large teeth are grooved If the Plectrodonts be jaws with anchylosed teeth, they belong to an order distinct from the Plagiostomi. If they should belong to any of the fishes indicated by the dorsal spines and shagreen skin, a combination of characters would be exemplified not known in other formations or in any exist ing fishes.
No detached teeth unequivocally referable to a plagi ostomous genus, nor any true ganoid scale of a fish, have yet been found in the formations that have revealed these earliest known evidences of vertebrate animals. What, then, it may be asked, were the conditions under which so immense an extent, as well as amount, of sediment was deposited—including chambered Cephalopods, Gastropods, Lamellibranchs, Brachio pods, various and large trilobitic and entomostracous Crustace ans, with Crinoids, Polypes, and Protozoa—that precluded the preservation of the fossilizable parts of fishes, if that class of vertebrate animals had existed innumbers, and under the variety of forms, comparable to those that people the ocean at the present day ? Bonitos now pursue flying-fishes through the upper regions of an ocean as deep as any known part of the Silurian seas of which the deposits afford an idea of greatest depth. If fishes of cognate habits with the present deep-sea fishes, under whatever difference of form such Silurian fishes may have been manifested, had really existed, we might reasonably expect to find the remains of some of the countless generations that succeeded each other during that vast and indefinite period, sufficing for the gradual deposition of sedi mentary beds of thousands of feet in depth or vertical thickness.