120 PALvEONTOLOGY. 



and is armed along the posterior edge ; the longitudinal ribs 

 are fine and numerous, but are constricted at intervals, as in 

 the genus Clenacanthus, and become subtuberculate at the 

 base. He deems them significant of a distinct genus of shark- 

 like fishes.* We may infer that there co-existed a larger and 

 more powerful predatory fish against whose attacks the 

 Onchus was thus defended. 



In the same old formation, with the dorsal spines of 

 Onchus, are found, in fact, petrified portions of skin, tuber- 

 cular 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 

 in Silurian beds, 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 Sphagodits shagreen. Yet the denticles are confluent 

 with an outer ridge of the bone, according to the "pleurodont" 

 type, and consist of sejjarated large teeth, with minute serial 

 teeth in the interspaces ; and the large teeth are grooved 

 longitudinally.t 



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 



* In a formation in Indiana, United States of America, referred by Messrs. 

 Norwood and Dale Owen to the Silurian formation, a badly-preserved fossil 

 considered as an Ichthyolite, and referred to a genus allied to Pterichthys has 

 been discovered, and called Macropetalichthys raphiidolabis. (Silliman's 

 Journal, 1846, p. 367.) 



March 1857, p. 288, pi. x., figs. 2-4. 



