12 



MAEINE EEPTILES OF THE OXFOED CLAY. 



bone share in the formation of the antero-ventral portion of the facet, as is also 

 the case in the atlas and axis of Trinacromerum (Dolichorhyncops) figured by 

 Williston *. In Peloneustes the rib-facet is borne on the centrum of the axis alone, 

 the second ventral wedge-bone being much smaller than in Pliosaurus. The posterior 

 face of the centrum of the axis is evenly concave, with a well-marked pit in the 

 middle. 



The remaining cervical vertebras (text-fig. 2) are about eighteen in number. Their 

 centra are characterised by their shortness, the length in the mid-ventral line, where 

 they are thickest, being less than half the vertical diameter of their articular faces. 



Text-fig;. 2. 



Cervical vertebra of Pliosaurus ferox : A, from front ; B, from left side. (E. 3536, | nat. size.) 

 a.z,, anterior zygapophysis ; c.r., cervical rib ; n.sp., neural spine ; p.z., posterior zygapophysis. 



These are nearly circular in outline, the transverse diameter being a little longer than 

 the vertical ; beneath the neural canal the upper border is slightly concave ; the 

 surfaces are gently concave, with a median pit of such depth that the anterior and 

 posterior pits can only be separated by a very thin layer of bone, but in no case have 

 they been seen actually to communicate with one another. On the ventral side the 

 articular faces have a well-defined rim or border, which in some cases may form a 

 slight hypapophysial prominence, which looks as if it may be formed by a now com- 

 pletely fused intervertebral wedge-bone. The neural surface is slightly concave from 

 side to side ; the facets for union with the pedicles of the neural arch are diamond- 



* Op. tit. 



