﻿LIASSIC FORMATIONS. 



23 



At the fourteenth cervical the length of the centrum is 1 inch 5 lines ; that of its 

 pleurapophysis is 1 inch 9 lines ; the fore-and-aft extent of the base of the neural 

 spine is 1 inch 2 lines ; the height of the spine is 1 incli 6 lines, and its thickness is 

 3 lines. The total height of the vertebra is 4 inches. These dimensions are gained 

 by gradual increase from the tenth vertebra. In the nineteenth cervical the length of 

 the centrum is 1 inch 6 lines, the space between the pleur- (ib. pi)) and neur- (ib. np) 

 apophysial surfaces is 7 lines. From the lower part of the centrum to the summit of 

 the neural spine ( ns) is 5 inches ; the length of the pleurapophysis is 2 inches. 



The Plesiosa/ov/s rostratus ranks with the section of its genus characterised by 

 broad and short cervical vertebrae. The instructive characters derivable from this 

 region will here be described as they appear in the fifteenth of the series (Tab. X, 

 figs. 1 — 3). This vertebra gives the following dimensions : 



In. lines. 



Length of centrum 16 



Height of terminal surface of ditto, or vertical diameter .... 1 7 



Breadth of ditto 2 6 



Breadth of the middle of centrum ........ 2 3 



From the under part of centrum to the summit of neural spine ...40 



Fore-and-aft extent of neural spine at its middle ..... 1 2 



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



of the other 2 4 



,, „ neural arch below the zygapophyses ... 1 1 



,, „ costal surface . . . . . . . . 10 



From the costal surface of the base of the neurapophysis .... 8 



The terminal articular surface of the centrum is nearly flat, very slightly convex 

 towards the circumference, and similar^ concave at the centre • it is transversely 

 elliptical, with a rather thin border, pretty closely co-adapted to that of the contiguous 

 centrum. The sides of the centrum are moderately concave, the under surface is more 

 deeply so ; and this is further excavated on each side of an obtuse median ridge (r), 

 near which the venous canals open into the large and deep ellipsoid fossae. The outer 

 boundary of these fossae is formed by the lower border of the costal articular surface 

 (Tab. X, fig. 3, and Tab. XI, fig. 2 pi). The costal surface (Tab. X, fig. \, P i) pre- 

 sents an oval form, with the long axis parallel with that of the centrum, 10 lines in 

 length by 8 lines in breadth, situated at the angle between the lateral and inferior 

 surfaces, and divided by a smooth, non-articular trait of the lateral surface, of 8 lines 

 in vertical extent from the neurapophysial surface of the centrum; this is defined 

 below by a slightly curved subangular border, convex downward. The fore surface 

 of the centrum presents a slightly fibrous character, not so smooth as in some other 

 species, nor so irregular as in the PI. rvgosns, for example. 



The neural arch is broad and low ; the zygapophyses project from nearer the base 



