88 PALEOZOIC FISHES OF NORTH AMERICA. 
eye will easily discover differences in the groups which are arranged strati- 
graphically, a predominating type of form or markings associating those of 
the Devonian with each other, and separating them in a rough way from 
those of the Carboniferous age. Still, the type which is predominant in the 
Carboniferous occurs in the Devonian, where may be also found as excep- 
tions the smooth-ridged species of the Mesozoic. That there were strongly 
marked differences between the fishes that carried teeth so much alike is 
quite certain, for the group designated by the name of Dipterus is so abun- 
dant and so well preserved in the Devonian rocks of Scotland that its entire 
structure has been fully made out, and we find that it was a fish having a 
tessellated cranium, the palate teeth already described, and a fusiform body 
covered with strong, enameled, punctate scales. In the Carboniferous 
and still higher strata, on the contrary, the fishes which carried the fan- 
shaped dental plates must have been somewhat differently constituted, for 
neither in the Old nor in the New World has anything like the com- 
plete form of the fish been made out. In the lagoons of the coal-marshes 
of England and Ohio, where the circumstances were favorable for the 
preservation of even delicate structures, the teeth, usually dismembered, 
but occasionally attached to the palato-pterygoid and the splenial bones, 
and portions of the tessellated cranium, were the only parts preserved; while, 
as yet, in the higher strata nothing but the teeth have been found. This 
is, however, an indefinable difference, and much more material than we yet 
possess must be obtained before we can satisfactorily coordinate the fossil 
Dipterine fishes among themselves or demonstrate their collective or indi- 
vidual relations to the living Ceratodus. This has been attempted by Hux- 
ley, Gunther, Miall, Traquair, and others, but further than demonstrating the 
wide differences in structure between the Devonian and Carboniferous Dip- 
terines and the living Ceratodus their labors have thrown little light upon 
the classification of the various representatives of this great genetic line of 
fishes. For the present, then, it is safer not to attempt to classify accurately 
the similar teeth of Dipterus, Ctenodus, and the fossil Ceratodus ; but, as others 
have done before us, we now provisionally class all the older Dipterines as 
belonging to the genus Dipterus, the Middle and Upper Carboniferous species 
as Clenodus, the Triassic as Ceratodus; but it would be quite impossible to 
