﻿LIASSIC FORMATIONS. 



13 



neurapophysial surface. A low, longitudinal rising, or obtuse ridge, traverses the free 

 surface of the side of the centrum midway between the pleur- and neur-apophysial 

 articular surfaces. 



The following were dimensions of the centrum of a cervical vertebra? answering, 

 or nearly so, in position, to that of the Plesiosaurus ccelospondylus selected for measure- 

 ment — 



In. lines. 



Length . . . 2 



Breadth of the articular surface ...... 1 10 



Height of ditto ......... 1 6 



These dimensions showed the greater proportional length of the cervicals of the 

 present species ; and, concurring with the more obvious difference in this shape of 

 the terminal articular surfaces, I thereupon devised the name of Plesiosaurus homalo- 

 spowdyhis* indicative of the even or level character of those surfaces, for the species 

 so characterised. 



I have subsequently received several additional vertebral evidences of both 

 these species of the Upper Lias, or Alum Shale of Whitby, and, finally, have had the 

 opportunity of studying two almost entire skeletons of the Plesiosaurus homalospondylus 

 from that locality, one of which (Tab. VIII) is now in the Museum of the Philosophical 

 Society of York, and the other (Tab. V) has been purchased by the Trustees of the 

 British Museum, where it is now exhibited in the Geological Department. Both of 

 these specimens exhibit the striking character of the genus Plesiosaurus in a 

 maximised degree, viz., in the length of the neck and the smallness of the head. 



I propose, first, to describe the specimen in the British Museum, Tab. V. 



This specimen gives indications of the same conditions of interment in its matrix, 

 and of the operation of subsequent gradual pressure, as that of the species last described^ 

 from the lias of another part of the kingdom. 



It has sunk into the mud, which afterwards became petrified, either prone or supine ; 

 for I have been unable to obtain evidence as to whether the present exposed part 

 of the skeleton was wrought out from the upper or under surface of the block, as re- 

 moved from the quarry ; but we may assume the former, and consider that the 

 animal was originally imbedded with the upper or dorsal surface toward the observer. 

 Both fore and hind paddles were outstretched at right angles with the axis of the 

 trunk, but only their proximal bones or segments have been preserved. The skull 

 and cervical vertebrae have maintained their original position. At the base of the neck, 

 where the neural spines, from their height and breadth, began to afford a surface upon 

 which the dislocating force could operate, they have begun to yield toward the left 



* 'OunXo's, planus ; a-KovhvKoi, vertebra. 



