MUEJiNOSAURUS. 91 



anterior part and summit of the triangular coronoid process (text-fig. 48, spl.), and this 

 region may include the fused coronoid bone, which has not been seen as a separate 

 element in this genus, although it may be present in Tricleidus. The line of junction 

 with the conjoined surangular and articular runs downwards and forwards from a 

 point a little behind the summit of the coronoid process. 



The posterior part of the mandible appears to be composed of only two separate 

 pieces — one, the fused articular and surangular (PI. VI. fig. 2 ; text-fig. 48, s.ang. & 

 art.) ; the other, the angular (ang.). The combined surangular and articular form the 

 upper and posterior part of the hinder end of the ramus ; anteriorly the surangular 

 unites in suture with the coronoid region of the splenial ; below this on its outer face 

 it bears deep depressions (text-fig. 48, s.d.) which receive the upper part of the 

 posterior end of the dentary ; below it unites with the angular in a long suture, Avhich, 

 as far as a point beneath the articular surface, runs parallel with the lower border 

 of the jaw. At this point the suture turns up, and in the postarticular region runs 

 along the upper and outer edge. The relations with the angular cannot be clearly 

 seen on the inner face of the jaw, at least not posteriorly. The articular surface is very 

 deeply concave from before backwards (more than a semicircle in Af. platyclis figured 

 in text-fig. 48) ; in its outer half it is bounded anteriorly by a very strong prominence 

 projecting backwards and inwards. The surface, though concave from before back, is 

 slightly convex from side to side, the median ridge thus formed working in the groove 

 between the outer and inner condyles of the quadrate. No line of separation between 

 the articular and surangular has been seen in any of the specimens examined. The 

 angular {ang.) is a very large bone forming the whole of the lower part of the posterior 

 half of the mandibular ramus. Its suture with the surangnlar and articular has been 

 described above. In the postarticular region it forms a thin plate on the outer side 

 and also the lower border and lower part of the inner face of the articular-surangular, 

 but here the sutures are obscure. Possibly the inner half of the surface for the 

 quadrate may be in part formed by the angular. In front of this surface the bone is 

 deeply grooved longitudinally, the groove (text-fig. 48, g.) extending right forwards to its 

 anterior extremity, where it receives the lower edge of the dentary, which overlaps 

 it also on the outer side of the jaw, the surface of union being distinctly marked 

 (text-fig. 48, s.d.). 



The above description diff'ers in several respects from that given by Williston * in 

 the case of the mandibles of some American Plesiosaurs. In the mandible of 

 Polycotylus, for instance, he recognises the presence of a suture between the articular 

 and surangular, and also the occurrence of a prearticular element. He also, both in 

 this genus and in the closely allied Trinacromerum f , finds a distinct coronoid element, 



* " North American Plesiosaurs : Trinacromerum" Journal of Geology, toI. xvi. (1908) pp. 720-1, fig. 6. 

 t " North American Plesiosaurs, Pt. I.," Field Columbian Museum, Geological Series, vol. ii. no. 1 (1903) 

 pp. 29-32, fig. 31. 



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