618 GEOLOGY OF OHIO. 



• primeval sharks as the comrades or the antagonists of the primeval 

 Pteraspidians of the Silurian seas. 



The genus Ckdodus was founded by Agassiz in 1843 to receive cer- 

 tain teeth consisting of a median cone on an elliptical base with one or 

 more lateral denticles on each side, the largest of which (if several were 

 present) were at the two extremities. The tooth of the species described 

 by Dr. Newberry was of this character, with however only a single denti- 

 cle on each side, as shown in Plate XI/VI. The second specimen, figured 

 as C. Fyleri, shows no teeth and was referred to the genus from its general 

 resemblance to the former. 



One of the specimens found by Dr. Clark is of a very peculiar form, 

 and allowing for a considerable change due to the conditions of fossiliza- 

 tion which may have somewhat modified its original outline, it yet mark- 

 edly differs from Dr. Newberry's species. It is shown on Plate 43, fig. 6. 

 Its reference to the genus Cladodus must be regarded as merely provi- 

 sional as no teeth have been found with it or on it. The head is wide 

 behind and narrower in front. It . was comparatively a small fish, not 

 measuring more than tweuty-eight inches in length, allowing for the in- 

 complete condition of its posterior end. The body is thicker in propor- 

 tion in the middle than is usual with sharks and tapers rapidly behind. 

 The pectoral fins are less straight in outline and less rigid than in C, 

 Kepleri. They contain about seventeen or eighteen rays the larger of 

 which fork near the tip. The margin is membranous. 



At its hinder end the body or rather probably its superficial skin is 

 extended into a wide membranous sheet lying horizontal, rather abrupt 

 in front and tapering away behind till it merges in the median extension 

 of the body carrying the caudal fin. This fin is not shown but the bases 

 of two distinct bony rays are seen indicating its presence. These spi- 

 nous rays are better shown in some of the other species. 



No ventral or anal fins are visible, they having been apparently of a 

 very soft and perishable nature, differing much in this respect from the 

 powerful and thick-rayed pectorals. Traces of the jaws and of the 

 branchial arches remain. The shagreen covering the body is well pre- 

 served in many p'aces. No trace of a dorsal fin or of any fin-spines can 

 be detected. The dorsal surface is shown for the most part except where 

 the stone flaked so as to expose the lower aspect near the head. There 

 is consequently no appearance of the scaly ventral defensive skin, that is 

 seen on these fossils when the lower aspect is exposed. 



For this species the name Cladodus? sinuatus is proposed, on 

 account of the sinuous outline of the head as it is preserved in the fossil. 



Another of Dr. Clark's specimens shows in spite of the necessary 

 indistinctness of a pyritized fossil, points of divergence so strongly ac- 

 centuated as to render it doubtful if it can rightly be referred to the same 

 genus. Yet in other respects it so clearly resembles the above as to for- 

 bid the drawing of any very strong line of demarcation between them. 



