XII. 



PLESIOSAURUS. 



313 



PLESIOSAURUS PLICATUS. Phil. 



In the Oxford clay begins a series of plesiosaurians with, trans- 

 versely elliptical articulating faces to the vertebrae, especially the 

 cervical and the anterior dorsals. Two species are known in the 

 Oxford clay, others in the strata above, which differ in the relative 

 lengths of the bodies. In no case, as yet, is the head recognized. 



In the Oxford clay the species belonging to the * ellipsospon- 

 dylian' race are known only by vertebrse and ribs, though it may 



Diagram CXVIII. Cervical Vertebrae of Plesiosaurus plicatus, seen sideways. 

 Scale, one-fifth of nature. 



be possible hereafter to assign to these some of the limb-bones. If 

 not identical with species in the Kimmeridge clay, they are very 

 closely allied to them. 



The faces of these vertebra are so very gently concave as in some 

 cases to appear almost plane; neatly bordered, wider than high, 



1 2 3 



Diagram CXIX. Cervical Vertebrae of Plesiosaurus plicatus. 



Scale, one-fifth of nature. 

 i. Seen from behind. 2. Seen from below. 3. Seen in front. 



broadest below the centre, with marginal parapophyses. The sides 

 are impressed above the parapophyses ; the base is plano-concave, 

 with two parallel depressions, and a pair of approximate large 



