574' 



Mr. D. M. S. Watson on some 



whole structure is very similar in the two types. The sur- 

 angular and angular are flat bones overlapped by the dentary 

 in front and covering the outer side of the small articular 

 and tlie long prearticular. The articular has two cotyli for 

 the double condyle of the quadrate, and has a small post- 

 articular process which is directed backwards and slightly 

 downwards. 



Pelycosauria. 



The lower jaw of Dlmetrodon is now well known from the 

 work of E. G. Case; its structure is shown in fig. 1, which is 

 diagrammatized from his published figures and photographs, 

 checked by specimens at Munich and Tubingen. 



Fiff. 1. 



A. Left ramus of the lower jaw of Dimetrodon, inner aspect. B. Right 

 ramus of the same jaw, outer aspect. Teeth omitted. X \. 



Ang., angular ; Aet., articular ; Ch., corouoid : Den., dentary ; Pb.Art,, 

 prearticular, Williston, = dermarticular, Kingsley, = gouiale, Gaupp ; 

 Sp., splenial; Sub.An»., surangular. 



The large dentary meets its fellow in a loose symphysis, 

 and at the back overlaps the surangular and angular. These 

 latter are thin flat bones covering the outer faces of the 

 articular and prearticular. The angular is remarkable for 

 possessing a notch on the lower border towards the back ; it 

 wull be shown in this paper that this notch more or less 



