162 Dr. C. W. Andrews — Skull and Jaw of Peloneuxtcx. 



Dolichorhynchops osborni and Brachauehenius lucasi, both from the 

 Cretaceous beds of Texas, describes the parietals as separating the 

 frontals, and extending between them to meet the posterior ends of 

 the facial processes of the premaxilloe. Possibly in these cases also 

 the overlap of thp parietals may be mainly superficial, and the 

 frontals actually extend back to the level of the parietal foramen, in 

 front of which they may either meet in the middle line on the inner 

 surface of the skull, or at least be separated by a small interval only. 

 If this is the case the structure of the skull in these American 

 Plesiosaurs would fall into line with that seen in the European forms. 

 The Leeds Collection contains- some very complete specimens of the 

 mandible of Pliosaurus and Peloneustes, and examination of these, 

 particularly of some belonging to Peloneustes philarchus, shows firstly 

 that the structure of this part of the skeleton differs considerably 

 from that occurring in the Elasmosaurs, and secondly that the 

 description of the Elasmosaurian mandible given in the first part of 

 the Catalogue of the Marine Reptiles of the Oxford Clay is wrong 

 in several particulars, and I take this opportunity of making the 

 necessary corrections. 



sang art g- 



ang 



Fig. 2. Inner face of the left ramus of the mandible of (A) Peloneustes 

 philarchus (about one-eighth nat. size) and (B) of Murcenosaurus (a smaller 

 specimen), ang. angular ; cor. coronoid ; d. dentary ; d.sym. symphyses 

 of dentaries ; g. dental groove ; s.ang.art. united surangular and articular ; 

 spl. splenial ; spl.sym. symphyses of the splenials. 



Each half of the mandible of Peloneustes is composed of five pieces. 

 These are (1) the fused articular and surangular, (2) the angular, 

 (3) the dentary, (4) the coronoid, (5) the splenial. Of these, as 

 usual, the dentary (d.) is much the largest, extending from the coronoid 

 angle to the anterior end of the jaw. It bears about forty teeth, 

 of which about fourteen are in the symphysial region. The whole 

 symphysial surface is formed by the dentary except a narrow ventral 

 strip (s.sym.) formed by the extension forwards of the splenials. The 

 anterior ends of the coronoids, though just extending into the 

 symphysis, do not seem to have actually united with one another 

 in the middle line. On the outer face of the jaw the posterior end 

 of the dentary overlaps the surangular above and the upper part of 



