METEIOEHTNCHUS. 



165 



circular in outline ; a very small portion of its upper lateral borders is formed by the 

 bases of the neural arch. The posterior surface is slightly concave and somewhat oval 

 in outline. Ventrally the centrum is strongly concave from before backwards and 

 convex from side to side, there being a blunt longitudinal ridge. The centrum of the 

 second sacral is very similar. 



The neural arch bears a high spine (n.sp.) much thickened at the summit ; ventrally 

 it unites with facets occupying the anterior two-thirds of the centrum, and also with 

 the upper surface of the inner end of the sacral rib, apparently sending out a process 

 along the dorsal surface of the latter, as in Steneosaurus and Mycterosuchus (text-fig. 51). 

 The zygapophyses are well developed in both sacrals. 



The sacral ribs (s.r., text-figs. 63, 64) differ from those of Steneosaurus in being more 

 slender and in cxirving downwards to a greater extent, so that their outer ends are con- 

 siderably below the level of the ventral face of the vertebral centra. At their inner ends 

 they unite with the bases of the neural arches and the sides of the centra ; externally 



Text-fig. 64. 

 a.c 



Sacral vertebrae of Metriorliynchus moreli, from below. (E. 2504, i Eat. size.) 



a.c, anterior face of sacrum ; il.g., surface for ilium ; p.c, posterior face of sacrum ; s.r., sacral ribs. 



(The centrum of the first sacral vertebra has been shortened through crushing.) 



the ends of the two ribs meet in a small triangular surface and, for union with the 

 ilium, bear concave facets, which look outwards and downwards. 



The caudal series (text-figs. 65, 66) includes a very large number of vertebra?, which 

 differ greatly from one another in different parts of the tail. The greatest number 

 of caudals preserved in any specimen in the Museum is thirty-five. Arthaber mentions 

 that in the different specimens of Metriorliynchus from Peterborough, preserved in 

 various Continental museums, the number varies from thirty-three to thirty-six, and 

 that in a specimen in Vienna the flexure of the tail occurs at the twenty-fifth vertebra, 

 while in one at Munich it is at the twenty-sixth. Probably, with the small terminal 

 caudals, the tail consisted of about forty vertebrae in all, the bend occurring at the 

 twenty-fifth or twenty-sixth. Geosaurus is described by Fraas as possessing forty-four 

 caudals. 



