No. 382.] DINICHTHYID OSTEQLOGY. 757 
figures, given by this author, leads us to believe that the central 
was in reality undivided in Brachydirus; that is to say, its 
relations are the same as in Coccosteus and D. pustulosus. 
There is a chance, therefore, that the recognition of a pre- 
central plate in Dinichthys depends upon faulty observation, 
and we are strongly of the opinion that no Jdona-fide sutures 
were ever seen on the dorsal surface cutting off a ‘portion of 
the central, as earlier writers would have us believe. But on 
examining the visceral surface of the head shield, one can easily 
understand how the solidifying crescentic ridge (seen one on 
either side of the median line and abutting against the equally 
heavy ridge of the median occipital), which stands in marked 
contrast to the thin forward extension of the central, might give 
one the impression of a distinct element.! The function of 
these ridges is to strengthen the base of the skull, and it would 
be strange, indeed, if they were cut through by sutures visible 
from below, where such are generally more obliterated than on 
the dorsal surface, and yet are not apparent from above. It is 
true that the ridges rise very abruptly, but although their 
separation from the central plate proper may seem to be indi- 
cated by some specimens, owing to difference in texture of 
bone substance, we regard it as very improbable that a suture 
exists here. Inasmuch as the central is a single element in 
D. pustulosus, it would certainly be anomalous not to find it 
entire in all species of this genus. 
The median occipital element of D. intermedius is acutely 
pointed in front, with the apex extending forward between the 
centrals, as in Phlycteenaspis and Brachydirus, but this is an 
exception to the general rule in Dinichthys and Titanichthys. 
The anterior margin of the median occipital in D. terreili 
resembles that of D. pustulosus, except that the indentations 
are shallower. Both D. intermedius and D. terrelli have the 
nuchal margin strengthened below by a heavy ridge which 
extends from the median line obliquely outward and backward 
on either side as far as the sockets of the exoccipital plates. In 
D. pustulosus the ridges on the under surface of the occipital 
1 Cf. Newberry, J. S. Palzozoic Fishes of North America, Monogr. U. S. Geol. 
Surv., vol. xvi (1889), Pl. LII, Fig. 1. 
