CRETACEOUS PLESIOSAURS. 217 



end than in the typical Plesiosaurian teeth. These specimens were found in the lowest 

 bed of the Lower Green-sand beneath Shanklin Chine, Isle of Wight ; I am indebted 

 for the drawings of them to John Edward Lee, Esq., of the Priory, Caerleon, Mon- 

 mouthshire. 



VERTEBRA OF A PLESiosAURUs. ' Enaliosauria,' Plate 27. 



The subject of the above-cited Plate 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 Flesiosauriis, with which the 

 structure of the neural arch is conformable. 



The followin"- are the chief dimensions of this vertebra. 



a 



Inches. Liues. 



Antero-posterior diameter of the centrum .... 2 2 



Transverse diameter of its articular cud ..... 3 



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



This vertebra differs from that of the Plcsiosainifs Ber/uirdi, not only in ihe 

 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 tlie present vertebra might be one nearer the back than the vertebra 

 figured in PI. 26, at which part of the spine the cervical ribs increase in size, have 

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

 the species in which they become anchylosed in the more advanced vertebr;c, there 

 would still remain the following difi'erences : — the vascular foramina on the under 

 surface are not situated in such deep and wdl-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. jNIy present experience of the constancy of such 

 secondary characters in the cervical vertebnc of the same species of Plcsiosanrtiff, leads 

 me to conclude that the vertebra figured in PL 27 is of a distinct species of Vlcxio- 

 saurm from that figured in PI. 26, a conclusion to which we are also led by the 

 consideration that the vertcln'al l)odies usually gain in breadth as they approach the 

 back, whilst the vertebra, (PI. 26,) with a lower placed rib, is relatively broader than 

 the present one. From the FJcHiomnruH pachi/omns, from th(> (Jrccn-sand of Ileaeli. 

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



* Report on Britisli Fossil Reptiles, IS.'i!), p. 71. 



