LIASSIG 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 inch 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.jjZ)) and neur- (ib. wp) 
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 Plesiosaurus rostratiis 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 : 
lu, lines. 
Length of centrum ........... 1 6 
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 . . . . . . . . 0 10 
From the costal surface of the base of the neurapophysis .... 0 8 
The terminal articular surface of the centrum is nearly flat, very slightly convex 
towards the circumference, and similarly concave at the centre ; it is transversely 
elliptical, with a rather thin border, pretty elosely co-adapted to that of the contiguous 
centrum. The sides of the eentrum 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. \, pi) 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 surfaee, of 8 lines 
in vertieal 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 PL ruc/osus, for example. 
The neural arch is broad and low j the zygapophyses project from nearer the base 
