Reptilian Lower Jaws. 517 



the wliole of which is covered by the angular. Posteriorly it 

 unites by a suture, which is very seklom visible, with the 

 articuhir. A good deal of its inner surface is covered by the 

 prearticular. 



The angular is a very large bone, consisting essentially of 

 a more or less flat plate tightly applied to the outer sides of 

 the snrangular and prearticular and separated above and 

 behind by a suture from the suranguhir. Its anterior end is 

 bevelled off to a point and received in the groove between 

 the dentary and spleuial. This plate is, however, modified 

 by a great development of the notch mentioned above in 

 JJimetrodon, protected and covered by a strong reflected 

 lamina from the outer face of the bone, which forms a pocket 

 open backwards. The body of the bone and the reflected 

 lamina combine to form a deep descending flange on the lower 

 border of the jaw. 



The articular is perhaps the most characteristic bone of the 

 whole skeleton of Dici/nodon. Its articular face is directed 

 upwards and backwards; it is throughout divided by a median 

 keel which fits into a groove in the pulley-shaped quadrate. 

 The anterior part of the articular tace is shallowly concave 

 and narrow from side to side ; the concave region passes 

 quite smootlUy into a very convex (antero-posteriorly) surface, 

 the sides of which are bounded by upstanding parapets, which 

 render the bone much wider in this region ; the outer of 

 these is much the higher and clasps the side of the quadrate. 

 Posteriorly the articulating face is bounded by a continuation 

 of the edges of these parapets. The whole arrangement 

 permits a great backward and forward motion of the jaw, 

 together with an alteration of the angle which the upper 

 surface of the dentary makes with the [jalate even when the 

 mouth is closed. 



The whole structure is quite unique, but very characteristic 

 of Anomodonts, occurring as it does in a less pronounced 

 form in Endotlviodon, 



The hinder end of the articular is continued into a post- 

 articular process which is directed downwards and lies 

 entirely in front of the hinder end of the articulating surface. 

 It is still fairly large, and its flat posterior surface would 

 afford good attachment to the digastric muscle. Tliis process 

 is continued forward by a ridge which forms a very narrow 

 keel on the under surface of the body of the bone and is 

 connected with the prearticular. 



The prearticular is a long narrow bone, united by a suture, 

 which is seldom visible, with the inner side of the keel on 

 the under surface of the articular, and stretching forward on 



