160 PALEOZOIC FISHES OF NORTH AMERICA. 
opposite dentary bone to form a forked extremity to the under jaw; upper 
margin of the anterior half of the dentary bone set with strong, conical, 
acute, incurved teeth, about fourteen in number, which diminish in size as 
they ascend the elevated point; five or six conical, recurved teeth are set 
on the inner side of the triangular extremity of the mandible, filling the 
space between the point and the symphysis; a broad, roughened depression 
or pit at the symphysis marks the place of attachment of a strong ligament 
which united the mandibles; the posterior extremity of the dentary bone is 
flattened, spatulate, and straight. 
The above description was based on the anterior half of a dentary bone 
found in 1877 by Mr. Jay Terrell in the Cleveland shale in Lorain County, 
Ohio. After that time no traces of this remarkable fish had been met with 
until 1886, when Mr. Terrell obtained, in the same formation and in neigh- 
boring localities, two complete dentary bones, right and left, of nearly the 
same size. These are twelve inches long, but differ slightly in proportions, 
and evidently were derived from two individuals. No other portion of the 
bony structure of Diplognathus has been obtained, unless it shall prove that 
these jaws belonged with the plates described elsewhere in this memoir and 
called Glyptaspis. But few of these plates have yet been found, and it is 
evident that, like Diplognathus, Glyptaspis was a rare fish in the water basin 
in which the Cleveland shale was deposited. 
Although so anomalous in their structure, it is apparent that the jaws 
described above belonged to a fish that was a member of the family of the 
Dinichthidz, since in many respects they resemble the jaws of the different 
species of Dinichthys, Titanichthys, and Trachosteus, i. e., they are alike in 
having the posterior extremity flattened and spatulate, evidently once en- 
tirely buried in integument, while the anterior and exposed half is more 
massive, is composed of denser tissue, and rises to a pointed extremity some- 
what in the style of a sled-runner. In Dinichthys the anterior extremities 
of the mandibles were much more abruptly curved upward, and served 
simply as powerful, penetrating and grasping teeth. As in Diplognathus the 
extremities were divergent, but were not provided with teeth on the inside. 
Much remains to be learned in regard to the armor of Glyptaspis, since 
only a small number of the plates composing it have been discovered, but 
