106 



MAEINE EEPTILES OF THE OXFOED CLAY. 



the posterior portion of the tail are curved backwards, their outer ends being occupied 

 by a nearly flat facet. 



In both the cervical and caudal regions the ribs may fuse with the centra or remain 

 separate. Their freedom or fusion is not of any systematic significance, except so far 

 as it may indicate that in some forms the adult condition is attained while the animal 

 is small, while in others the fusion does not take place till it has reached a large size. 

 The same remarks hold good with regard to the fusion or separation of the neural 

 arches throughout the column. 



The arrangement of the ventral ribs (text-fig. 60, A, B) is not well known in this 

 genus: probably they were placed much as in Cri/ptocleidus (see below, p. 175) — at 

 least the individual elements are closely similar to those found in that genus, in which 



Tentral and dorsal ribs of Murcenosaurus durohrivensis : A, median ventral rib ; B, posterior median 



ventral rib ; C, dorsal rib from behind ; D, articular end of dorsal rib. (E. 2S63, ^ nat. size.) 



s.r., surface for union with lateral ventral rib. 



each of the transverse rows (with the possible exception of one or two posteriorly) 

 consists of a median and three pairs of lateral bones. The median bone (text-fig. 

 60, A, B) is overiapped at either end by the inner ends of the first lateral pair, which 

 are closely applied to its anterior face : these again are overlapped in a similar way by 

 the second pair, and these again by the inner ends of the outer pair (see figure of 

 plastron of Ciyptocleidus, text-fig. 86, p. 175). 



Shoulder-girdle (PL VI. figs. 3, 6 ; text-figs. 61, 62, 67, 68).— Before describing the 

 structure of the shoulder-gii-dle in Muramosaurm it may be well to give a brief 

 account of this portion of the skeleton in the Sauropterygia generally, especially as its 



