82 CONTRIBUTIONS FROM WALKER MUSEUM 
There was no sign of the lower jaws with the skull of Dimetrodon 
gigas, but with the other skull, No. roo1, the jaws were preserved 
nearly perfectly. They show that the portion identified by Baur and 
Case as the articular region of the skull is in reality the articular 
region of the lower jaw. The articular is small and nearly inclosed by 
other bones. Its upper face is marked by two deep cotyli, and the pos- 
terior edge in specimen No. 1oo1 has a small hook-shaped projection. 
The quadrate is supported by the angular, surangular, and splenial 
(Baur), prearticular (Williston). The posterior ends of these bones 
stand out from the thin expanded posterior end of the bone, supporting 
the articular bone on a sort of pedicel instead of on the upper edge of 
the jaw. This explains why the articular region is so often found 
isolated in the fossil beds. The posterior portion of the jaw is very 
thin, but expanded vertically. In both jaws the coronoid bone is 
lost, but it was a small, thin plate, as shown by the sutures for its 
attachment. Anteriorly the angular passes far forward, forming the 
posterior half of the outer side of the jaw. The splenial or prear- 
ticular reaches nearly to the middle of the jaw, where it disappears 
under the splenial (presplenial of Baur). The spenial reaches to 
the symphasis, but does not take part in it. 
As previously described, there are enlarged incisor and canine 
usks in the upper jaw, and enlarged incisors in the lower jaw. In 
Dimetrodon gigas the edges of the teeth are crenate, but this and the 
number of the teeth in the jaws seem to be somewhat variable in the 
different species of the genus. 
In general, the whole skull may be said to bear a remarkable 
resemblance to the skull of Sphenodon, in most parts being directly 
comparable to it, and varying only in the temporal arches—the ossified 
interorbital septum, and the vertical plates of the upper side of the 
pterygoids. 
By. (Cy GASn: 
STATE NORMAL SCHOOL, 
Milwaukee, Wis. 
