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 Otenacanthus 
wrightt, in the Hamilton; and forms like Helodus gibberulus in the 
Chemung indicate that the divergence of the Cochliodont branch took 
place at a period considerably antedating the Carboniferous. As the 
group of Cladodont sharks, which is remarkable for its manifold vari- 
eties of piercing teeth, frequented the clear water of open seas and was 
undoubtedly of carnivorous 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 are not morphologically very different from sharks. 
A much depressed form of body is indicated by the arrangement of teeth 
in such forms as Psammodus, Copodus, and Archeeobatis from the Car- 
boniferous, and Janassa from the Permian.t 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 were 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 origin 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. Without 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 
Edestus vorax, HE. minor, E. giganteus, etc. This difficulty has been 
1 On the form of body in Janassa and other Petalodonts, cf. Jaekel, O., Ueber 
die Organisation der Petalodonten. Zeitschr. deutsch. geol. Ges., Vol. LI., 1899, 
pp. 258-298, Pl. xiv., xv. 
