Dr. C. W. Andrews — Skull and Jaw of Peloneustes. 163 



the angular below the complex suture between them running down- 

 wards from the coronoid angle. The lower part of the angular is 

 prolonged forward beneath the dentary, forming the ventral border 

 of the jaw, to a point a little behind the symphysis. On its inner 

 face the upper part of the dentary is covered to a large extent by 

 the closely adherent coronoid, which extends from the coronoid angle, 

 in the formation of which it shares, to just within the symphysis, 

 though, as already noted, it is doubtful whether the coronoids 

 themselves actually united with one another. The coronoid {cor.) 

 is in turn itself overlapped on its inner face by the splenial (spl.), 

 which roofs in the dental groove (#.) and unites by its ventral edge 

 with the upper border of the angular, fitting into a deep groove 

 in the upper border of that bone posteriorly and overlapping it 

 anteriorly, where it runs down and forms the ventral border of 

 the jaw. At its anterior end the splenial extends some distance 

 in to the symphysis, uniting with its fellow in the middle line 

 (spl.sym.). At its posterior end it forms the inner and lower 

 edge of the opening of the dental canal (g.), and extends back as 

 a narrow strip of bone along the upper edge of the angular to a point 

 only a little in front of the articular surface for the quadrate. The 

 surangular and articular (s.ang.art.) are represented by a single 

 element : this forms the upper part and the rounded posterior end of 

 the post-articular process : it also bears the articular surface for the 

 quadrate, consisting of an outer and an inner concavity. In front of 

 this the upper border rises towards the coronoid angle, and is broad 

 and flattened. At the coronoid angle the bone is overlapped on its 

 outer side by the posterior end of the dentary, and on the inner side at 

 the same level it joins the coronoid ; the lower edge is united with 

 the angular throughout. The angular (ang.) forms the whole of the 

 lower part of the posterior half of the mandible, and it sends forward 

 a prolongation, overlapped externally by the dentary and internally by 

 the splenial, to within a few centimetres of the symphysis. Its upper 

 edge is deeply grooved and forms the floor of the dental canal, and 

 receives the lower edge of the splenial at least in the posterior portion 

 of that bone. 



In a species of Plesiosaurus from the Uitenhage Beds of South 

 Africa the general arrangement of the elements of the jaw are very 

 similar, except that the coronoid is much shorter and does not reach 

 the symphysis, and it is doubtful whether the splenial does so either. 

 In the Liassic Plesiosauria also the structure, so far as can be made out, 

 is similar: in the longer-snouted forms at least the splenial extends 

 into the symphysis. In the jaws of the Elasmosaurs of the Oxford 

 Clay the case is different, and it is necessary here to correct some 

 errors in the account of the structure of the lower jaw in these 

 reptiles, given in part i of the Catalogue of the Marine Reptiles of the 

 Oxford, Clay, examination of the Pliosaurian mandibles and other 

 material having thrown new light on the subject. In the account 

 there given the coronoid is said to be entirely wanting, while the 

 splenial extended from the coronoid angle to the symphysis, closely 

 adherent to the inner face of the dentary above the dental groove. 

 As a matter of fact the coronoid (Fig. 2, B), though lost in nearly all 



