LIASSrC PLESIOSAURS. 



35 



characterised by intermediate proportions of the centrum, neither "long," as in PL 

 homalospondylus, nor " short," as in PL rostratus and PL planus. The following are 

 the dimensions of this vertebra : 



Length of centrum ......... 



Height or vertical diameter of terminal surface of ditto . 

 Breadth of ditto 



,, the middle of centrum ....... 



From the under part of centrum to the summit of neural spine 

 Fore-and-aft extent of neural spine at its middle .... 



,, „ „ arch from the end of one zygapophysis to that of 



the other ..... 



,, ,, „ arch below the zygapophyses 



„ ,, costal surface ...... 



From the costal surface to the base of the neurapophysis 



The free or non-articular surface of the centrum shows near the margins of the 

 terminal surfaces the strongly marked rugous character which originally suggested 

 the specific name in the detached vertebrae. The irregular risings of bone lie chiefly 

 in the direction of the axis of the vertebra, and project so as to come into view exterior 

 to the articular surface in an end-view of the centrum, as in Tab. XV, fig. 2. The 

 sides and under part of the centrum are moderately concave lengthwise. The contour 

 of the terminal articular surface is circular; its border is thick and convex, leading to 

 a moderate concavity, with the central part rising into a slighter convexity. The costal 

 pits {pi) are of a full elliptical form, with a slightly prominent margin, situated near 

 the lower surface of the centrum, a little nearer the hind than the fore end, and with 

 twice their own vertical diameter intervening between them and the base of the 

 neurapophysis {np)- This part of the side of the centrum is traversed by a low, longi- 

 tudinal rising, a little nearer the neur- than the pleur-apophysis. The venous orifices 

 on the under surface of the centrum (Tab. XV, fig. 4) are two and a half lines apart, 

 with an intervening low, obtuse, longitudinal ridge, and are not situated in definite 

 depressions. The lower border or base of the neurapophysis has not the angular form 

 seen in PL Hawkinsii and PL dolichodeirus, but is curved. The neurapophysis rises, 

 with both fore and hind borders vertically concave, transversely convex, about five lines 

 above the centrum before giving off the anterior zygapophyses {:) ; the posterior 

 arcs [z') come off, as usual, a little higher. The neural spine is subquadrate, more 

 angular between the fore and upper margins than in PL rostratus (Tab. X, fig. 1); 

 the thickness of the spine is shown in Tfib. XV, fig. 2 ; the front border is sharp. The 

 chief variety observable in cervical vertebrae of the present species is the presence 

 of the longitudinal groove bisecting the costal surface. The articular surface of the 

 centrum in every cervical vertebra where it is exposed in the present skeleton repeats 



