OPHTHALMOSAIJEUS. 



45 



(text-fig. 27, E, F) in the region of the tail-fin the arches become rather higher and more 

 compressed, the spine sloping slightly backwards. The canal is small and is higher 

 than it is wide ; its lower portion is enclosed between the projecting processes of the 

 centrum, with which the arch articulates. 



The dorsal ribs (text-fig. 31, C, D) are long and comparatively stout. At the 

 proximal end the capitular and tubercular facets are Avidely separated, the forking of 

 the upper end being deeper than in most Liassic forms. The capitular facet (h.) is 

 the larger and nearly circular in outline, while the smaller tubercular facet {t.) is 



Text-fiff. 31. 



A. 



B. 



Sections of vertebral centra and the upper end of a dorsal rib of Ophtlialmosaurus : A, vertical section 

 through uncrushed vertebral centrum ; B, vertical section through centrum of a crushed vertebra 

 belonging to the same individual as last ; C, posterior face of upper end of dorsal rib ; D, anterior 

 face of ditto. (A, B, E. 2180 ; C, D, E. 2137, f nat. size.) 



7(., head of rib ; t., tubercle of rib ; s., roughened ridge for muscle-attachment.' 



somewhat compressed from before backwards, its anterior border being nearly straight. 

 The body of the rib is compressed from before back, but is so thickened along its outer 

 side that both the anterior and posterior surfaces present the appearance of being 

 grooved, the concavity being most marked on the posterior surface. On this surface 

 near the upper end of the rib the upper border bears an elongated roughened surface 

 (s.) for muscle-attachment. Towards their ventral ends the ribs become more rounded 

 in section. 



In the caudal region the single-headed ribs become reduced to mere nodules of bone 



