14 



MAEINE REPTILES OE THE OXFOED CLAY. 



The caudals (text-fig. 3), which are about 25-30 in number, have the centra rapidly 

 shortening as the series is followed backwards, and the last four or five diminish in 

 size very suddenly, the terminal centrum being a small irregularly-shaped nodule 

 of bone, which, however, seems to have borne a rudimentary neural arch, but, so far as 

 can be made out, neither caudal ribs nor chevrons. In the anterior caudals the 

 articular surfaces of the centra are nearly circular in outline and are gently concave 

 with a deep median pit. The neural surface (n.c.) is narrow, particularly in the 

 middle where it is encroached upon by the deeply concave facets for the neural arch 

 (a./'.). The sides of the centra are concave in a longitudinal direction ; on either side 

 they bear a short prominence (r.f.) with the summit of which the single-headed caudal 

 rib articulated or fused : the nutritive foramina are situated just beneath these costal 

 prominences. The ventral surface of the centrum is only slightly concave in a longi- 

 tudinal direction ; on its anterior and posterior edges are the semicircular chevron- 

 facets (c.f.), the anterior looking downwards and forwards, the posterior, the larger of 

 the two, downwards and backwards. 



Centrum of a caudal vertebra of Pliosaurus fero.v, from behind and from the left side. (E.3536, i nat. size.) 



a.f., facet for union with neural arch ; c.f., facets for chevrons ; n.c, floor of neural canal ; 



r.f., facet for rib. 



Bibs (text-fig. 4). — So far as has been observed, there is no rib on the atlas ; that on 

 the axis is not well known, but seems to have been single-headed and to have articulated 

 with a large facet borne mainly by the axis and partly by the second intervertebral 

 wedge-bone and the odontoid. This facet shows no trace of division into an upper and 

 a lower portion ; in Peloneustes the rib of the axis is double-headed and borne by the 

 centrum of the axis only. Behind the axis all the cervical ribs (text-fig. 4) are double- 

 headed, with the possible exception of one or two posteriorly : the upper (diapophysial) 

 head (t.) being separated from the lower (parapophysial) (h.) by a well-marked cleft 

 which is continued on to the anterior and posterior face as a short deep groove. The 

 distal end of the cervical rib is strongly compressed from above downwards and is 

 produced backwards into a short angle (p.). The upper side of the proximal surface 

 is roughened as if for the attachment of muscle, and from this point a ridge runs 



