Io6 REVISION OF THE PELYCOSAURIA. 



premaxillaries backward and downward in a curve to a point opposite the end of the 

 palatine. Their form and relations are shown in plate i8, figs, i and 2, and plate 

 19, figs. 2 and 4. The curvature of the lower surface makes a vaulted roof to the 

 mouth in the anterior portion. In about the middle of their course they are free from 

 the bones on the sides, leaving a cavity which forms the posterior nares ; the sides of the 

 prevomers at this point are marked by a prominent rugosity of the edge. Superiorly 

 and posteriorly the prevomers join the vertical pterygoid plates; superiorly the upper 

 edges diverge and receive between them the united plates; posteriorly they shade 

 indefinitely into the plates, so that it is impossible to fix the exact limits of the bone. 

 The lower jaw : In specimen No. looi the lower jaws are preserved almost per- 

 fectly ; the coronoid, which was a small splint bone, seems to be lost from both sides. 

 The posterior portion of the jaw becomes very high by the development of the posterior 

 bones as vertical plates and from the inner side of this region the articular region 

 projects as an almost sessile process made up of various processes from the angular, 

 suraugular, and 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 such an isolated mass which was interpreted by 

 Baur and Case as the articular region of the skull. Plate 9, figs. 1-4, shows the lower 

 jaws and the articular region in detail. 



The articular is a flattened disk-like bone completely inclosed 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 posterior end 

 of the surangular. This appears largely on the upper surface and fonns the inner 

 side of the pedicle supporting the articular and its main attachment to the main por- 

 tion 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 this bone and the 

 surangular. There is a deep pit extending backward and inward along the line of the 

 articular-surangular .suture. From posterior edge of articular in specimen No. lOOi 

 a curious short curved process extends inward and upward (plate 11, figs. 9-1 1). 



The main portion of the jaw is best understood from plate 9. 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 

 indicating the loss of the coronoid. 



The angular 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 (presplenial of Baur). 



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

 jaw, but does not take part in the symphysis. 



The dentarv carries a variable number of teeth in the different 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 tusks. 



