FISHES OF THE CARBONIFEROUS SYSTEM. lee 
that they should be referred to the sides of the head, and that their supposed 
places were really occupied by oblong or elliptical thinner plates which 
were partly covered with integument, and which have none of the incised 
lines that characterize all of the cranial plates. 
In D. Terrelli the suborbital plates are sometimes eighteen inches in 
length and eight inches wide, oblong in outline, rounded behind, and with- 
out any evidence of contact with other plates. Hence it is not strange that 
they were considered the homologues of the ‘“post-ventro-lateral” plates of 
Coccosteus. In D. intermedius, however, they are much shorter, and closely 
resemble in form and markings the suborbital bones of Coccosteus. They 
show, too, the incised lines which are wanting on the other bones of the 
plastron and are traceable on all the cranial plates. It is a singular fact 
that the pattern formed by these lines is the same in general plan in Di- 
nichthys and Coccosteus, and is practically alike on the suborbital bones of 
Dinichthys, Coccosteus, and Titanichthys. ence the positicn of these subor- 
bital plates in the cranium now figured would seem to be normal, and we 
must replace them by others in the posterior part of the plastron. That 
other plates did occupy this position is proved by the rhomboidal expansion 
of the posterior end of the sternal plate, which by its beveled margins shows 
that it was overlapped by other plates on all sides. Since the bones of the 
plastron are always separated, we can only be guided in its reconstruction 
by finding places for all the cranial and dorsal plates, so as to complete the 
defenses of the upper side of the body, and then distribute the plates which 
covered the under surface according as they adapt themselves to those of 
which the places are known. The plates of the under side of the body were 
relatively thin, devoid of surface ornamentation, and were probably to a more 
or less degree covered with an integument. Of these, a pair which when 
united formed by their outlines a Gothic arch, I have supposed to be jugular 
plates, which filled the space between the mandibles. One of these is rep- 
resented by outside and inside views, half size, on Pl. VI, Figs. 1,1. They 
are each semi-elliptical in outline, sixteen inches in length by seven and a 
half wide; the anterior ends are pointed; the outer margin is symmetrically 
arched; the inner margin nearly straight; the posterior ends are obliquely 
truncated and overlapped by the anterior extremity of a second pair of 
