OSTEOLOGY OF THE SKULL OF THE DIMETRODON 311 



There was no sign of the lower jaws with the skull of Dimetrodon 

 gigas, but with the other skull, No. looi, 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. looi has a small hook-shaped projection. 

 The quadrate is supported by the angular, surangular, and splenial 

 (Baur), prearticular (Wilhston). 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 

 tusks 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 Spkenodon, 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. 



E. C. Case. 



State Normax School, 

 Milwaukee, Wis. 



