FISHES OF THE DEVONIAN AGE. 55 
before me seem to establish beyond question the position of these teeth in 
the symphysis of the jaw. 
The pattern formed by the numerous plates that compose the cranium 
of Onychodus is very complicated, and presents a puzzle not yet solved, for 
the cranium has never been found entire; and indeed it has rarely happened 
that any two plates have been seen in connection. In this we have evidence 
of a want of solidity in the structure of the cranium, such as has not been 
noticed among any of the congeners of this fish. A number of the plates 
which compose the bony structure of the head can, however, be easily iden- 
tified, such as the opercula, parietals, frontals, ete , and all the plates which 
belong to Onychodus are easily recognized by their peculiar tuberculation. 
This is relatively fine, and may be compared to a series of small, striated, 
appressed cones. In some instances these cones show but a single furrow 
by which they are notched at the summit ; their height is two or three times 
the diameter, but each is laid over and adherent to the plate by its entire 
side. A similar tuberculation, though finer, covers the exposed portions of 
the mandibles and maxillaries. 
In regard to the affinities of Onychodus, it is impossible now to speak 
with absolute certainty, as all the specimens yet found have been in such a 
dismembered condition as to leave some important points of structure unde- 
termined. There is little doubt in my mind that it belonged to the Cross- 
opterygide. he features which lead to this conclusion are the cycloidal, 
imbricated scales, having the exposed portion strongly, the covered portion 
more delicately, yet elegantly, ornamented, much as in Glyptolepis; the spatu- 
late or sandal-form jugular plates, and the large rounded opercula. Both 
these latter plates, as well as the supra-temporals, resemble in form those of 
Polypterus. Hence we may infer that when the structure of Onychodus is 
more fully made out, we shall find that the paired fins were more or less 
lobate, the body fusiform, and the general appearance not unlike that of 
Holoptychius and Glyptolepis. 
The peculiar dentition I have described is a point in the structure of 
Onychodus where it differs widely from the fishes with which I have asso- 
ciated it; but I have elsewhere shown how much the dentition varies among 
both recent and fossil fishes, which by other characters are somewhat 
