42 



MAEINE EEPTILES OF THE OXFOED CLAY. 



to the greater transverse convexity of the ventral surface. Above the rib-facet the 

 sides are somewhat flattened and the neural surface is still nearly as broad as in the 

 dorsal region; the rounded and as it were unfinished edges of the centrum show that 

 a good deal of cartilage persisted in this region of the column, even in advanced life. 

 Posteriorly the tail seems to have narrowed very rapidly, and the centra become first 

 more rounded, then somewhat compressed laterally, so that they are deeper than wide, 

 and at the same time the rib-facets disappear. This continues till two or three 

 vertebrae of peculiar form (text-figs. 27, C, D, and 28, A, B) are reached. These seem 

 to mark the point where the downward flexure of the column occurs ; their centra are 

 much deeper than wide and at the neural border are considerably longer than at the 



Text-fig. 28. 



n.c ,„/ 



Caudal vertebrse of Ojphihalmosaurus : A, centrum of vertebra at bend of tail from front ; B, ditto from side ; 



C, posterior caudal (some distance behind bend) from front ; D, ditto from side. (E. 2169, | nat, size.) 



c./., facets for chevrons ; n.c, floor of neural canal ; n.f., facets for neural arch. 



ventral. The concavities of the articular faces of the centra are surrounded by a 

 broadly convex border ; the middle of the depression bears a deep pit, marking the 

 position of the notochord. The neural canal here forms a narrow deep groove, with 

 the facets for the neural arch raised considerably above its floor. Both in these 

 vertebrse and in a considerable number in front of them, the facets for the chevrons 

 are well marked. Behind the bend of the tail come a large number of vertebrae in 

 which ossification seems to be much more extensive (text-figs. 27, E, F, and 28, C, D) 

 than in the anterior caudals, all the borders and surfaces being, as it were, neatly 

 finished and sharply defined : this may result from the circumstance that these 



