764 THE AMERICAN NATURALIST. (VOL XXXII. 
Newberry’s figures show the formation of the nuchal sockets 
on the under surface of the occiput very clearly. They are 
quite deep and divided by a thin longitudinal septum, but are 
not bounded below by a transverse septum, as in D. terrelli. 
The thickness and compact texture of the bone substance in 
this region are very remarkable, as noted above. The nuchal 
ridges are broad and massive, but not nearly so prominent as 
in Dinichthys; and the median longitudinal ridges, together 
with those belonging to the central (“ parietals”), which are 
so conspicuous in D. zerrelli and D. intermedius, are here alto- 
gether lacking. 
The arrangement of sensory canals is sufficiently indicated 
by the diagram, and the articulation of cranial and dorsal 
shields is so familiar from Newberry’s writings that we may 
pass over these topics. We cannot agree with the latter 
author, however, that the hinge joint permitted a lateral as 
well as vertical motion of the head shield, and even the vertical 
motion must have been restricted in large measure by the over- 
lapping claviculars. 
The outline of the dorso-median is reduced from a photo- 
graph purporting to be of T. clarkii, but, as will be shown 
presently, there are good reasons for believing it to belong to 
this species. Not only does it overlap the inner edges of the 
dorso-lateral plates (for a distance not determinable from the 
specimen at hand, but probably greater than shown in Fig. 4), 
but it in turn passes underneath a heavy flange which is 
given off from the superior surface of the dorso-laterals. The 
edges of the dorso-median are thus received into a deep groove 
formed by the side plates; and in another species, as we shall 
see, the articulation was still further complicated. How far 
the flange extended backward over the surface of the dorso- 
median cannot be told with certainty, as it is broken off in the 
manner shown by Newberry’s illustrations. 
The term dorso-lateral is here used to include the mass of 
bone contiguous to either side of the dorso-median, the com- 
ponents of which are apparently fused. That two elements 
are concerned in the formation of this apron-like expanse 1S 
patent from a number of features, such as the arrangement of 
