100 MARINE EEPTILES OE THE OXEOED CLAT. 



Behind the base of the sacral ribs the edges of the centra are strongly concave from 

 before backwards. The neural arch unites with the centrum and the upper part of the 

 base of the sacral rib, sending out a short lateral process along it. The anterior 

 zygapophyses are fairly large and resemble those of the dorsal vertebra?. They are 

 separated anteriorly by a deep fossa for the attachment of ligament, which is continued 

 up the anterior edge of the neural spine as a roughened groove. The laterally 

 compressed neural spine is broad and curved slightly forwards ; its upper end is abruptly 

 truncated and somewhat thickened. The posterior zygapophyses seem to have been 

 small, but are not well preserved in any specimen. 



The second sacral vertebra has the anterior articular surface smaller than the 

 posterior, and nearly circular in outline. The base of the sacral rib is attached to the 

 middle portion of the centrum, not extending to the articular face at either end. The 

 ventral surface of the centrum between the bases of the sacral ribs is convex from side 

 to side and strongly concave from before backwards. As in the dorsal vertebrae, the 

 edges of the centra are thrown into numerous plications. The posterior articular face 

 is gently concave ; in St. durolrivensis it is nearly as high as wide, but in St. leeclsi the 

 transverse diameter is considerably the greater. The neural spine is similar to that of 

 the first sacral, but the posterior zygapophyses are better developed. 



The number of caudal vertebrae (text-fig. 38) is uncertain, none of the series 

 preserved being complete. In the type specimen of St. durolrivensis there are 38 or 

 39, but the terminal vertebrae are wanting. In a specimen of St. leeclsi (R. 3806), in 

 which also several terminal caudals are missing, there are 36. 



In all the skeletons the anterior one or two caudals seem to be missing — at least, 



none showing the peculiar characters seen in the anterior caudals of Mycterosuchus 



nasutvs (see p. 137) have been observed. In the remainder of the caudal region the 



various species differ considerably in the form of the vertebrae, especially, as in the 



case of the cervicals, in the length of the centrum and the form of the neural spine. 



These differences, which are correlated with the form of the skull, will be referred to 



under the several species. Speaking generally, the caudals in this genus have the 



centra much constricted in the middle, while at the same time the ventral surface is 



flattened, or even concave, from side to side ; in the anterior portion of the tail the 



ridges separating the ventral from the lateral surfaces of the centra are truncated at 



their posterior ends by oblique facets for the chevrons, which seem to have little or no 



contact with the anterior face of the succeeding centrum. The articular ends of the 



centra vary considerably in form ; thus in St. durolrivensis they are gently concave 



throughout the series, while in St. leedsi the faces of the anterior caudals are very 



gently concave, but in the posterior region they tend to form a convex border round 



a central concavity. The outlines of the articular faces vary in different parts of the 



column. The vertical is always greater than the transverse diameter and, in the 



region where the chevrons occur, the form is somewhat barrel-shaped, the sides being 



