EASTMAN : CARBONIFEROUS SHARKS. 71 



forms related to Campodus, this family makes its first appearance in the 

 Lower Devonian of Canada and Great Britain. That it had attained 

 considerable specialization at least as early as the Mesodevonian, is 

 proved by the occurrence of formidable fin-spines, such as Ctenacanthus 

 wrighti, in the Hamilton ; and forms like Helodus gibberulus in the 

 Chemuns indicate that the divers^ence of the Cochliodont branch took 

 place at a period considerably antedating the Carboniferous. As the 

 group of Cladodont shai'ks, which is remarkable for its manifold vari- 

 eties of piercing teeth, frequented the clear water of open seas and was 

 undoubtedly of cai-nivorous habits, so, on the other hand, the groups 

 armed with crushing teeth, such as are typified by Psammodont, Coch- 

 liodont, and Cestraciont sharks, early became adapted to bottom-living 

 conditions, their fare probably consisting of hard-shelled prey such as 

 mollusks, arthropods, and echinoderms. In all likelihood it is to the 

 generalized Cestraciont type that we must look for the derivation of 

 rays, which after all ai-e not morphologically very diflFerent from sharks. 

 A much depressed form of body is indicated by the arrangement of teeth 

 in such forms as Psammodus, Copodus, and Archpeobatis from the Car- 

 boniferous, and Janassa from the Permian.-^ The Devonian Tamiobatis 

 is held to represent an intermediate type between sharks and rays ; 

 hence there is considerable reason to suppose that the modern ray-type 

 was foreshadowed at even so remote a period as the Devonian. 



Form and Orientation of Segments. — Interesting inquiries might be 

 made respecting the mode of growth of the series in these four related 

 genera, and into the processes of segmentation and fusion of the individual 

 teeth ; but we can only briefly touch upon these topics in the present 

 paper. That various speculations have been entertained as to how the 

 successional teeth wei'e developed in Edestus, and that confusion still 

 exists in the case of some species, regarding which are the oldest and 

 which the newest formed segments, cannot be gainsaid. Dean's theory 

 of a metameral oi'igin for these bodies, and all others which fail to rec- 

 ognize their odontological nature, are of course to be dismissed in the 

 light of our present information. AVithout a knowledge of the arrange- 

 ment of the symphysial teeth in Campodus and Helicoprion, the orienta- 

 tion of incomplete series would still be conjectural in many cases, such 

 as in the species of Campyloprion just described, and the types of 

 Mdestiis vorax. E. minor, E. giganteus, etc. This difficulty has been 



^ On the form of body in Janassa and otlier Petalodonts, cf. Jaekel, 0., Ueber 

 die Organisation der Petalodonten. Zeitschr. deutsch. geol. Ges., Vol. LI., 1899, 

 pp. 258-298, PL xiv., xv. 



