OF THK PELYCOSAURIAN GENUS DIMETRODON. 25 



gular ami prearticular (splenial) ; for this reason the posterior portion of the jaw is 

 almost always shattered in the ground and the more solid articular region is the most 

 commonly preserved. It was sucli an isolated mass which was interpreted by Baur 

 and Case as the articular region of the skull. 



Figs. 1 and In, PI. IIT, shows the lower jaws and the articular region in detail. 



The articular : The articular is a flattened di.sc-like bone completely enclosed on 

 all sides but the superior. The upper surface bears two cotyli corresponding to the 

 condyles of the quadrate. On the under side of the articular the posterior ends of the 

 prearticular (splenial) and the angular meet in the median line and furnish the main 

 support of the articular region ; between the articular and angular is slipped the pos- 

 terior end of the surangular, this appears largely on the upper surface and forms the 

 inner side of the pedicle supporting the articular and its main attachment to the 

 main portion of the jaw. On the outer side of the upper surface the prearticular 

 appears and the articular sends a process forward for a short distance between 

 tins bone and the surangular. There is a deep pit extending backward and in- 

 ward along the line of the articular-surangular suture. From the posterior edge of 

 the articular in specimen 1001 a curious short curved process extends inward and 

 upward. 



The main portion of the bone is best understood from figures. The articular 

 pedicle is crushed down, in the natural condition it stood out almost at right angles 

 from the jaw. 



The surangular passes directly into a broad plate forming the posterior portion of 

 the upper half of the bone ; it rises rapidly as it passes forward to meet the rising end 

 of the dentary. There are impressions on the adjoining ends of these two bones indi- 

 cating the loss of an element, the coronoid. 



The anrjular forms the lower portion of the posterior half of the jaw ; it is rather 

 wide and continues the lower edge of the jaw as far downward as the coronoid carried 

 the superior edge upward. It extends forward past the middle of the jaw forming a 

 good portion of the outer surface of the jaw. 



The prearticular extends forward between the angular and surangular till it 

 meets the splenial. 



The splenial is relatively narrow, covering the upper half of the inner face of the 

 jaw and extending as far forward as the symphasis of tlu; jaw but does not take part 

 in the symphasis. 



The dentary carries a variable number of teeth in the ditterent species, there are 

 always one or two enlarged tusks near the anterior end, corresponding to the incisor 

 tusks of the premaxillary above but none that correspond to the canine tusk. 



