164 PALEOZOIC FISHES OF NORTH AMERICA. 
shielded by bony plates. We can imagine that Diplognathus, if swift in its 
movements, might have found parts of the body of Dinichthys that were 
penetrable by the points of its mandibles, but Mylostoma was provided with 
no means of offense, and, unless armor-clad, would have had no safety but in 
flight. 
The plates of Glyptaspis which we have found are often broken as 
though by violence, even where composed of dense, bony tissue half an 
inch or more in thickness, and the surfaces of the plates, at least in one case 
which has come under my observation, show deep furrows, that have been 
traced by the points of the premaxillaries or mandibles of Dinichthys. 
My.ostoma TERRELLI, Newb. 
Plate XIV, Figs. 1, 2. 
Mylostoma Terrelli, N.; Trans. N. Y. Acad. Sci., vol. 2, 1883, p. 147. 
Principal inferior dental plates in pairs, each of which is spatulate in 
outline, with one margin nearly straight where it joined its fellow, the other 
strongly arched; length six to seven inches by two inches in greatest 
breadth; crown composed of dense, ename!l-like tissue eight lines in thick- 
ness at the front and gradually thinnin gtoward the narrow posterior end; 
triturating surface punctate or vermicularly roughened, slightly arched from 
front to rear, and rising into a low rounded boss near the external margin, 
where the tooth is broadest, and about one-third the length of the crown 
from its anterior extremity. The crown is supported below by a strong, 
bony keel, which begins at the anterior fourth of the length and gradually 
descends backward until it has a width of two and a half inches, terminat- 
ing ina thin irregular margin twelve to fifteen inches from the anterior 
extremity of the crown. 
Of this large species only a single specimen has yet been discovered. 
This includes the crown complete with a part of the splenial (?) bone on 
which it was supported. ‘The entire dentary bone must have been fifteen 
to eighteen inches in length. 
This specimen is figured on Pl XIV, views of both the side and crown 
surfaces being given. 
