PLAGIOSTOMI 101 



inhabited the sea-bottom in company with the Onchus-fish. 

 No vertebras, 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 Sphagodtts 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 

 longitudinally.* 



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 in numbers, and under the variety 

 of forms, comparable to those that people the ocean at the 

 present day 1 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 



* Egerton, Proc. Geol. Soc, March 1857, p. 288, pi. x., figs. 2-4. 



