102 MARINE REPTILES OF THE OXFORD CLAY. 



convex, the neural and ventral borders nearly straight; towards the hinder part of the 

 tail the degree of lateral compression of the centra increases. Caudal ribs (transverse 

 processes, c.r.) are confined to the anterior 12-14 vertebrae, decreasing in size from 

 before backwards. They are compressed from above downwards and curve a little 

 downwards ; they are borne wholly or, at least, mainly on the neural arch. 



The neural arches are supported by a long base which extends nearly from one end 

 of the centrum to the other. The anterior zygapophyses (a.z.) are borne on the front of 

 the arch ; they project sharply upwards and forwards beyond the articular surface 

 of the centrum, and their articular facets are inclined to one another at a very acute 

 angle. The posterior zygapophyses (p.z.) are borne at the base of the hinder border 

 of the neural spine ; they are separated by a deep pit, and inclined to one another at 

 a very acute angle. Both the anterior and posterior zygapophyses decrease in size as 

 they are followed back in the series, and finally disappear some distance from the end 

 of the tail. The neural spines differ much in the different species. In St. leedsi the 

 thin but broad spine is comparatively low ; in the anterior caudals it slopes a little 

 forwards, but further back comes to incline backwards. At about the middle of the 

 tail it bears a projection on its anterior border separated from the zygapophyses by a 

 notch. Further back (text-fig. 38, D) it becomes much narrowed, nearly vertical, and 

 situated over the hinder part of the arch ; the anterior portion of the spine is 

 represented by a sharp ridge, rising a little at its anterior end. In St. durobrivensis the 

 spine is higher ; in the first few vertebra? it is inclined forwards, but further back it 

 slopes backwards. At about the junction of the middle and anterior thirds of the 

 tail the upper part of the anterior border of the spine is cut away, and this narrowing 

 is increased in the vertebras further back, till here also the spine becomes very narrow 

 and is situated mainly over the hinder part of the arch. 



Ribs (text-fig. 39). — In Steneosaurus durobrivensis (K. 3701) the first (atlantal) rib is 

 a straight bar of bone (text-fig. 39, A) ; at its wider anterior end it bears a flat facet for 

 union with the corresponding surface on the postero-inferior border of the anterior 

 wedge-bone (text-fig. 34). The outer face is convex in its anterior half, but flattened 

 posteriorly, where its lower edge forms a sharp and rather roughened ridge. In 

 St. leedsi the outer face is roughened throughout. The anterior part of the inner 

 face is concave from above downwards, apparently for apposition to the outer face 

 of the second (axial) rib ; posteriorly the inner surface is slightly convex. The rib 

 terminates in a blunt roughened point, probably tipped with cartilage in life. 



The bone here regarded as the second (axial) rib (text-fig. 39, B) is peculiar ; it is 

 very much smaller than the rib in front, and from its somewhat irregular form probably 

 undergoing reduction. At its anterior end it bears a roughened oblique facet for 

 union with the surface borne half on the odontoid and half on the axis (text-fig. 34). 

 Towards its posterior end it becomes strongly compressed and terminates in a point ; 

 on its upper border near the anterior end there is a sharp ridge which may represent 



