OPHTHALMOSAUEUS. 39 



The centnim of the third cervical (text-fig. 25) is, like that of the axis, somewhat 

 triangular, or perhaps rather pentangular in outline ; it narrows somewhat ventrally, 

 and bears on the mid-ventral line a fairly strongly-marked hypapophysial ridge. Both 

 the anterior and the posterior faces are deeply cupped, and, in the type specimen at 

 least, the whole of these faces, except a narrow rounded border, is concave ; so that 

 the statement that in this genus the concavity is confined to the central portion of the 

 centrum and is surrounded by a flattened area, is not correct, except in the case of 

 some specimens which have been somewhat crushed. The surfaces {n.f.) for union 

 with the neural arch are deeply concave, and extend from end to end of the centrum ; 

 in front they are connected laterally with the diapophysial surfaces. The parapophyses 

 are distinct tubercles, ending in a flattened or slightly concave surface ; in this region 



Centra of anterior cervical vertebra of Ophthalmosaunis, from left side. (R. 2135, f nat. size.) 



d., diapophyses ; n.f., facets for neural arches ; p., parapophyses ; w.h.f.^, w.l.f.^, facets 

 for the first and third subvertebral wedge-bones. 



they are situated not quite halfway down the side of the centrum and rather nearer 

 the anterior than the posterior border. The next few vertebrae are very similar, except 

 that at about the fifth their ventral surface becomes evenly rounded from side to side, 

 the hypapophysial ridge disappearing ; the degree to which this ridge is developed varies 

 considerably in diflferent individuals. As the vertebrae are followed backwards the 

 centra become more circular in outline, the surfaces for the neural arch become longer 

 and narrower, and the diapophyses separating from them (at about the 19th vertebra) 

 shift lower and lower on the side of the centrum. This downward movement is also 

 shared by the parapophyses, though to a less degree, so that about the 38th vertebra 

 the diapophysis joins the papapophysis, the resulting faces being situated low down 



