FISHES OF THE CARBONIFEROUS SYSTEM. 141 
cipital. In D. minor it seems to have been easily separable from the other 
portions of the cranium, for we have many disarticulated specimens of it. 
One of these is figured on Pl. VIII, devoted to that species. 
Between the supraoccipital and the marginal the space is occupied by 
an oblong and relatively thin plate, almost never separated from its asso- 
ciates except by fracture. This is the plate called by Dr. Traquair external- 
occipital, fully identified by its incised lines and by its articulation with 
the supra-scapula; it is called by Huxley the parietal-epiotic. In Coccosteus 
it is triangular in outline, but in Dinichthys it is narrowed behind by the 
angle of the head being brought so far backward. This is the plate with 
which the suprascapula articulates in the wonderful joint to which I have 
so frequently made reference. It is pierced at its inner posterior angle by 
a vertical, thimble-like socket, into which the condyle of the supraseapula 
fits in such a way that it cannot be withdrawn without being lifted vertically. 
Apparently to prevent the binding of this joint, a thumb-like guard is thrown 
out from the angle of the head and passes under the edge of the supra- 
scapula. 
The central portion of the head in Dinichthys is occupied by an irregu- 
lar plate, which is probably divided by a suture down the middle, though 
never separating along this line. It is identifiable by its relations to the 
surrounding plates and by the lines of ornament (essentially the same in 
both genera) with the plate in the cranium of Coccosteus called frontal by 
Huxley and central by Traquair. 
The defenses of the back of Dinichthys formed by the great dorsal plate 
and the supra-scapulas have been so fully described in the Report of the 
Geological Survey of Ohio, that litthke more remains to be said about them. 
The resemblance of the dorsal plate of Coccosteus to that carried on the 
back of some of the plated Siluroids has been remarked upon by Huxley, 
Owen, and Hugh Miller. The similarity is as marked in the dorsal of Dinich- 
thys. Its position and functions are not only the same, but it is supported 
by a process extending from the posterior margin down to the neural spines, 
just as the dorsal plates of the Siluroids are; so that there can hardly be a 
doubt of its being their homologue and an inherited character; one of the 
many which connect the Siluroids and the Placoderms. The form of this 
