CRETACEOUS FORMATIONS. 63 



VERTEBRA OF A PLESIOSAURUS. 



The subject of T. XIX is a mutilated vertebra, there figured of the natural size, 

 which was obtained from the Chalk-pit at Burham, in Kent, and is now in the 

 Collection of Mrs. Smith, of Tunbridge Wells. 



The centrum, slightly concave at both ends, with a large vertically oval depression, 

 fig. 3, pi, for a rib on each side, and with a pair of vascular foramina on its under 

 surface, fig. 2, c, c, shows the characters of the genus Plesiosaurus, with which the 

 structure of the neural arch is conformable. 



The following are the chief dimensions of this vertebra. 



Inches. Lines. 

 Anteroposterior diameter of the centrum .... 2 2 



Transverse diameter of its articular end ..... 3 



Vertical diameter of ditto ....... 3 



This vertebra differs from that of the Plesiosaurus Bernardi, not only in the 

 proportions indicated by the dimensions above given, but likewise by the non- 

 anchylosis of the rib, and by the shape and position of the surface for its attachment 

 to the centrum : and if the value of these differences were to be questioned on 

 the ground that the present vertebra might be one nearer the back than the vertebra 

 figured in T. XVIII, at which part of the spine the cervical ribs increase in size, have 

 their junction raised nearer to the neural arch, and retain longer their individuality in 

 the species in which they become anchylosed in the more advanced vertebrae, there 

 would still remain the following differences : — the vascular foramina on the under 

 surface are not situated in such deep and well-defined pits ; the concave terminal 

 articular surfaces have not the central depression : the sides of the centrum are not 

 bevelled off at the border of these articular surfaces, but are divided from them at a 

 right angle by a well-defined margin. My present experience of the constancy of such 

 secondary characters in the cervical vertebras of the same species of Plesiosaurus, leads 

 me to conclude that the vertebra figured in T. XIX is of a distinct species of Plesio- 

 saurus from that figured in T. XVIII, a conclusion to which we are also led by the 

 consideration that the vertebral bodies usually gain in breadth as they approach the 

 back, whilst the vertebra, (T. XVIII,) with a lower placed rib, is relatively broader than 

 the present one. From the Plesiosaurus pachjomus, from the Green-sand of Reach, 

 near Cambridge,* the present specimen differs in the form of its costal surface, which 

 is vertically instead of being transversely elliptical : it is still more obviously distinct 



* Report on British Fossil Reptiles, 1839, p. 74. 



