PELONEUSTES. 4i 



Its lower edge is overlapped by the splenial. The splenial (spl.) is a large plate of 

 bone which completely roofs in the dental groove from the level of the coronoid angle 

 to the symphysis, into the ventral side of which it extends for some distance (spl.sym.), 

 uniting with its fellow of the opposite side. For a short distance behind this the 

 splenial forms the lower border of the mandihular ramus ; it then overlaps and unites 

 by its lower edge with the angular, its union with that element extending to its 

 posterior end, which is only a little in front of the articular surface for the quadrate. 

 This posterior pi'olongation of the splenial behind the coronoid angle forms the lower 

 border of the opening of the dental canal (g., in text-fig. 14, B). The surangular and 

 articular (s.ang.ari.) are indistinguishably fused with one another. The articular 

 surface for the quadrate is divided into an inner area deeply concave, especially from 

 before backwards, and a smaller outer area, only slightly concave from side to side, but 

 deeply so from before backwards. The anterior border of the articular surface forms a 

 prominent lip, which added greatly to the strength of the joint. Just behind the outer 

 portion of the articulation there is a deep pit, presumably for a ligament. Of the 

 postarticular process the surangular and articular form the upper surface and also the 

 rounded angle. In front of the articulation, and in a line with its outer portion, the 

 upper edge of the surangular rises to the coronoid angle, and is much thickened and 

 flattened at the top. In front it unites in overlapping sutures with the dentary 

 externally and the coronoid internally ; it actually extends some distance in front of 

 the coronoid angle, but is concealed by these two bones. The lower border joins the 

 angular {ang.). This bone forms the whole of the lower part of the posterior region 

 of the jaw. In its posterior portion its upper surface is in close sutural union with the 

 surangular and articular, in front of the articulation the bone deepens vertically as far 

 as the level of the coronoid angle, and its upper surface bears a deep groove, the outer 

 side of which receives the posterior prolongation of the splenial. Anteriorly the 

 angular is prolonged forwards as a narrowing process to within a few centimetres of 

 the symphysis ; in this region it joins the dentary on the outer side of the jaw,, the 

 splenial on the inner, both these elements overlapping it. 



Vertebral Column (text-figs. 15-20). — -The cervical vertebra?, including the atlas and 

 axis, are about 21 in number. AH, with the exception of the atlas, bear ribs which are 

 double-headed, except perhaps in the case of the posterior one or two of the 

 series. The neural spines are high and narrow. 



The atlas and axis (text-figs. 15, 16) are closely united in full-grown specimens, but 

 the lines of division between the constituent parts usually remain more or less distinct. 

 In a young specimen (R. 2439, text-tig. 16} all the constituent elements are separate, 

 so that the structure of the atlas-axis complex can be clearly made out. The anterior 

 surface of the body of the atlas (od.) forms the middle and upper portion of the cup 

 for the basioccipital ; it is concave, and in its centre is a slight pit. Below its lower 

 third there is a deeply concave roughened surface for union with the anterior wedge- 



