﻿LIASSIO FORMATIONS. 



.1 



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, z> ) over- 

 hangs a small part of the end of the succeeding centrum, and the neurapophysis (ib., 

 fig. 4, n) 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, ?;s,Tab. Ill, fig. 4, ns) arises narrow between the bases or 

 back part of the prezygapophyses (z), and its base extends, increasing in thickness 

 gradually to near the back part of the postzygapophyses (-'). 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 by a rounded apex overhanging, in the foremost vertebras 

 (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, ns. 



The pleurapophysis of the axis (Tab. Ill, fig. 1, X pl) 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 o: 

 Bristol" (p. 559). 'Transactions of the Geological Society of London,' first series, vol. v. 



