524 ON A NEW SPECIES OF PLESIOSAURUS 



measurements are nearly equal, though the vertical predominates a 

 little. So far as they are visible in the transverse sections exposed by 

 fracture of the limestone slab, the articular faces of the centra are 

 nearly circular. 



7. The cervical costal pits are elliptical, about half as long 

 vertically as longitudinally, and from the third to the twenty-sixth 

 inclusive are divided lengthwise by a well-marked longitudinal depres- 

 sion ; but there is no subdivision into two distinct facets. In all 

 these vertebra; the pits look outwards and a little downwards, their 

 axes are parallel with those of the vertebra;, and they are completely 

 sessile. 



In the last three cervical vertebrae the costal pits are directed more 

 and more backwards as well as outwards, and take the form of flattened 

 facets. At the same time their anterior edges are raised up by an 

 ■outgrowth of the body of the vertebra. 



8. The articular facets of the anterior dorsal vertebrae are nearly 

 circular. In the anterior eight or nine dorsal vertebra; the transverse 

 processes arise partially from below the level of the upper margin of 

 the centrum. In the tenth they appear to arise completely above it, 

 their upper margins being on a level with the upper edges of the 

 posterior zygapophyses. In the eighteenth they begin again to 

 descend, so that in the first sacral more than half the root of the 

 transverse process is below the level of the superior margin of the body. 



9. The neural spines of the cervical vertebrae are inclined a little 

 backwards, and have their anterior edges bevelled, so that their apices 

 are more or less pointed. Those of the dorsal and sacral vertebrae are 

 vertical, with their anterior and posterior margins parallel and their 

 apices squarely truncated. 



10. The articular faces of the caudal vertebra; are nearly round, 

 and their centra larger vertically than longitudinally. The neural 

 spines slope backwards a little, but their anterior edges are straight 

 and their ends truncated. The three or four last caudals have 

 apparently neither spines nor neurapophyses. 



There are more than thirty named species of PLesiosaurus. Of 

 these, however, far more than half are founded upon detached bones, 

 and I am not aware that entire, or nearly entire, specimens of more 

 than four species, viz. dolichodeiriis, Hawkinsii, macrocephaliis, and 

 brachycephalus, have as yet been described. This point is worthy of 

 notice when we consider that the proportion of the head to the body 

 constitutes an important datum in the determination of the species of 

 this genus. I will compare P. Etheridgii first with those of which 

 complete or nearly complete skeletons have been observed. 



