174 



MARINE EEFTILES OF THE OXFOED CLAT. 



in height, the decrease continuing to the end of the tail. The zygapophyses are well 

 developed, somewhat concave (or convex) from side to side, and looking more inwards 

 (or outwards) than further forwards. In the caudal region (text-figs. 84, 85) the centra, 

 as already mentioned, are wider than deep ; laterally, at least in the young, they bear 

 prominent facets for union with the caudal ribs, which in the adult become joined to the 

 centra. These ribs are compressed from above downwards, and a little behind the middle 

 of the tail some are considerably expanded towards their outer ends ; in all there is a 

 tendency to curve backwards. The facets for the chevrons commence on the second 

 or third caudal ; at first they are confined to the hinder part of the centrum, where 

 they form a pair of projections truncated by a flat nearly circular surface looking 

 downwards and backwards. Further back (text-fig. 85) these facets are on both the 



Text-fig. 85. 



Posterior caudal vertebrae of Ci-yptodeidus oxoniensis : A, end of caudal series from left side ; 



B and C, posterior caudal vertebrae from below. (E.. 3705, | nat. size.) 



c.f., facet for chevron-bone ; c.r., caudal rib ; n.a., neural arch. 



anterior and posterior edges of the centra, the heads of the chevrons articulating 

 between the successive centra ; the posterior facet is the larger. The neural arches 

 on the caudal region decrease in height gradually from before backwards ; the zyga- 

 pophyses disappear about the middle of the tail ; the arches of the caudal vertebree are 

 the last to fuse with the centra, and the fusion takes place from before backwards. 



The chevrons are not well known ; in the young they are little rounded rods of bone 

 with a slightly expanded vertebral extremity. 



The cervical, sacral, and caudal ribs have already been referred to. The dorsal ribs 

 are thickened and oval in section towards their articular ends, which terminate in a 



