16 



MAEINE EEPTILES OF THE OXFOED CLAY. 



terminates in a slightly concave surface, in which the bone appears to be made of a 

 closely compressed bundle of individual bone-fibres. Evidently this was a sort of 

 growing point, and immediately above it the surface is perforated by many vascular 

 foramina which run upwards. Throughout nearly its whole length the rib is nearly 

 circular in section, there being no trace of grooving : moreover, it appears to have been 

 very heavy and solid in texture, somewhat like the rib of a Sirenian. 



The caudal ribs are not well known. They articulate with the centrum by a single 

 head, the surface of which is nearly flat. The body of the bone is compressed from 



Immature left coracoid of Pliosaurus ferox : A, from above ; B, symphysial surface. 



(E. 2738, about 1 nat. size.) 



a.p., anterior prolongation ; gl.c, glenoid cavity ; sc.f., facet for scapula ; sym., symphysial surface. 



above downwards and curves a little backwards ; the anterior and posterior edges seem 

 to have formed strong cristiform ridges. 



Shoulder-girdle (text-fig. 5). — No complete specimen of the shoulder-girdle is known, 

 but some isolated bones have been found and can be definitely determined as belonging 

 to Pliosaurus, because they are similar in form, at least so far as can be made out, to 

 the bones of the imperfect shoulder-girdle associated with the skeleton R. 3536, of 

 which the skull is preserved. 



