200 TJie American Geologist. October, i892 
cranium and shown the various plates, and to as great a degree as 
possible, the way and extent of their overlap and underlap, but 
in consequence of their co-ossification these are in a few places not 
exactly and positively determinable. The specimen figured meas- 
ures ten and a half inches from the tip of the nasal plate to the 
nuchal angle and the right side, which is scarcely displaced at all, 
is five and a half inches in breadth. 
The principal new details in the skull are in the forms of some 
of the plates and the over and underlap which has not previously 
been represented. 
Beside this, the structure of the upper jaw shows important 
additions, the principal of which will form the subject of a future 
•'note." 
THE PLATES OF THE HEAD. 
(1.) The Snpva-occipital occupies the middle part of the hind 
margin, and is very massive, being one inch and a quarter thick 
at the maximum, extending forward as shown and very slightly 
underlapping both the adjoining lateral plates. Its inner face 
carries three ridges, one running forward and the other two out- 
ward and backward along the margin. These latter are very 
thick and heavy, and their sutures with the ex-occipitals are nearly 
vertical, not underrunning the latter to any considerable extent. 
The sutures of the supra-occipital with both the parietal and 
ex-occipital or so nearly vertical that it is possible the overlap 
may be even the other way. This was certainly the case with 
D. minor whose supra-occipitals are often found separate. In 
these, the ex-occipital underlaps the supra-occipital, and the latter 
is considerably overlapped by the parietal. In direction the}' are 
nearly parallel with the axis of the head. 
At the junction of these three ridges and in the middle of the 
nuchal line is the well known double socket on the inner face so 
plainly shown in Dr. Newberry's figures. (See pis. iv and lii of 
Monograph). The function of this socket is not exactly known, 
but it probably marks the place of insertion of some powerful 
muscle or ligament that connected the head with the rest of the 
body. 
(2.) The Ex-occipital "^laXe. forms most of the hind margin of 
the head outside of the supra-occipital and carries the socket of 
that singular lock-joint with the supra-scapula which character- 
izes the Arthrodira. On the inner side it moderately overlaps 
