FISHES OF THE DEVONIAN AGE. 49 
a broad expansion of marginal wall, inclosing, perhaps, a pulp cavity, while 
the other is simply concave below, for adaptation to the convex surface of 
a supporting jaw. Just such a difference, indeed, is seen in the teeth of the 
upper and lower jaws of Callorhynchus. 
The microscopic structure of these teeth is similar to that of many of 
the teeth of extinct Selachians, such as Cochliodus, Psammodus, ete., viz, a 
cancellated or reticulated tissue formed by radiating and branching calcig- 
erous tubes completely solidified near the exterior, but wearing in such a 
manner as to leave a peculiar roughened and punctate grinding surface. 
The general form of the most perfect of these teeth is very similar to that 
of those I have designated by the name of Rhynchodus secans; but in that 
species the lower margin is not so strongly arched, and the crown forms a 
cutting edge which played on that of the opposite tooth. If my view that 
these are the teeth of Chimzroid fishes is accepted, it. will perhaps be 
thought that the differences between these two species have generic value, 
as they are quite as striking as those which separate the genera Hdaphodon, 
Ganodus, Ischyodus, Elasmodus, etc. Yet, as it seems to me, with so little 
material before us and knowing so little of the complete dentition of either 
species, it would be somewhat premature to attempt now to define more 
than one genus. 
The resemblance presented by these teeth to those described by Buck- 
land! under the name of Chimera Townsendii is so striking, that to any one 
who will make the comparison the question will seem to be uot so much 
whether the teeth under consideration are those of a Chimeeroid fish, as 
whether the present species and that of Buckland do not belong to the same 
genus. 
Ruyncuopus crassus, Newb. 
Plate XXVIII, Fig. 4. 
Rhynchodus crassus, Newb. ; Paleontology of Ohio, vol. 1, p. 312, pl. 29, fig. 3. 
Teeth large and strong, three to five inches in length, one and a half 
to two inches in breadth. Base expanded, with a somewhat semi-circular 
1 Proc. Geol. Soc. London, vol. 2, 1838, p. 206, 
MON XvI——4 
