LIASSIC FORMATIONS. 
0 
posterior two fifths of the centrum in advance ; the articular surface, the length of 
which equals two thirds that of the length of the centrum, looks obliquely upward and 
inward ; that of the posterior zygapophysis has the reverse aspect. As the cervicals 
approach the back the zygapophyses diminish in relative size, and their articular sur- 
faces become less horizontal. The posterior zygapophysis (Tab. Ill, fig. 4, ^') over- 
hangs a small part of the end of the succeeding centrum, and the neurapophysis (ib., 
fig. 4, rises with a deeper concavity at the back than at the fore part. That 
this anchylosis had not occurred in the similarly sized and in the larger specimens of 
the cervical vertebrae of the Plesiosaurus described and figured by Conybeare in his 
first famous Memoir^' is due to their having been derived from a younger specimen 
of a larger species from the Bristol Lias, probably Plesiosaurus arcuatus. 
The neural spine (Tab. I, HsyTab. Ill, fig. 4, ns) arises narrow between the bases or 
back part of the prezygapophyses (s'), and its base extends, increasing in thickness 
gradually to near the back part of the postzygapophyses (s')- The height of the spine 
averages half the vertical extent of the entire vertebra from its summit to the lower 
level of the centrum, being rather shorter in the anterior cervicals and exceeding that 
length at the base of the neck. In the anterior cervicals the contour of the neural 
spine extends from the fore part of the base, in a curve increasing in convexity at the 
upper part, and terminating hy a rounded apex overhanging, in the foremost vertebree 
(as at c4, Tab. Ill), the concave contour of the hinder border. The upper part of the 
spine becomes more squared as the spine itself gains in height, in the larger posterior 
cervicals, by the increasing fore-and-aft extent of their upper part, as in fig. 4, 7is. 
The pleurapophysis of the axis (Tab. Ill, fig. i, xpl) has its posterior angle 
extended backward ; that of the third cervical has its anterior angle also produced 
forward, but in a minor degree. Both angles continue to be more produced in the 
succeeding vertebrae, but the front one most so, until, in the fifth cervical, they are 
equal in length ; the hinder one then elongates, but they do not touch or overlap the 
contiguous pleurapophyses until about the tenth cervical vertebra. The extent of 
this terminally dilated or extended border of the riblet exceeds that of the diameter 
of the same from its upper articulation outward or downward. The line of articulation 
is discernible in most of the anterior vertebrae, but in fig. 1 coalescence has com- 
menced, if it be not complete, as in figs. 4, 5, 6, pi, in which the expanded part of the 
pleurapophysis has been broken off, showing the approximated head and tubercle 
adapted respectively to par- and di-apophysial divisions of the costal surface. In 
* “ Notice of the discovery of a new fossil animal, forming a link between the Ichthyosaurus and 
Crocodile, together with general remarks on the osteology of the Ichthyosaurus ; from the observations of 
H. T. de la Beech, Esq. F.R.S., M.G.S., and the Rev. W. D. Conybeare, F.R.S., M.G.S. (Read April 6th, 
1821.) Drawn up and communicated by the latter.” The observations on the vertebral characters of the 
new reptile are said to have been made “ on the organic remains contained in the Lias in the vicinity of 
Bristol” (p. 559). ‘Transactions of the Geologieal Society of London,’ first series, vol. v. 
