CRETACEOUS FORMATIONS. 13 



for the British Museum, and adopt Mr. Campiche's name, which has the priority 

 of publication, under the conviction of the specific identity of the vertebrae from 

 the two localities. 



All the mamillate vertebrae I have yet seen from the Cambridge Green-sand 

 indicate a Plesiosaurus not larger than that represented by the smallest of the 

 dorsal centrums from St. Croix. 



The cervical vertebrae (Tab. VI, figs. I — 4) shows a greater proportional trans- 

 verse dimension of the centrum than in the vertebra from the dorsal region (ib., 

 figs. 9, 10), the sides of the centrum are less concave (compare fig. 4 with fig. 11) ; 

 on the inferior surface the shallow impressions into which the vertical venous 

 canals open, are divided by a narrow ridge-like tract (ib., fig. 4). The neurapo- 

 physial depressions (figs. 1 and 3, n p) are broader than in the dorsal centrum, are 

 of a triangular form, and, as the intervening neural tract is of equal breadth (ib., 

 fig. 3), it is relatively larger than in the dorsal vertebra (ib., fig. II); the venous 

 foramina in this tract (fig. 3, ») are also wider apart. The costal surface (fig. 1, p i) 

 is large in proportion to the centrum, well defined, but not deep ; transversely 

 elliptic ; 9 lines in longitudinal by 6 lines in vertical diameter, and 3 lines distant 

 from the apex of the neurapophysial pit (np) : it is situated rather nearer the pos- 

 terior than the anterior part of the centrum, and its margin slightly projects from 

 the level of the non-articular surface of the centrum; the distance between the 

 inferior borders of the two costal pits (fig. 4, pi) is 10 lines. The terminal articular 

 surface (fig. 2) is less concave than in the Plesiosaurus Bernardi, and although 

 obtuse and convex at the circumference, is less thick or tumid there ; but the con- 

 spicuous and chief distinction is the well-defined mammillary eminence in the centre 

 of each of the terminal concavities. The following are dimensions of this centrum : 



In. Hues. 



Length 11 



Breadth 1 5 



Depth 14 



Figures 5 and 6 represent a vertebra of apparently the same individual from 

 the base of the neck, where the costal surface (Tab. VI, fig. 5, pi) has almost wholly 

 ascended from the centrum upon the neurapophysis (np), and is more prominent 

 than in the average cervical vertebrae. The under surface of the vertebra is not 

 excavated or ridged, and is very slightly concave lengthwise ; it resembles that of 

 the average dorsal vertebrae. The mamillate character of the terminal articular 

 surface is as well marked as in the average cervical vertebrae. 



Figures 7 and 8 are of a posterior cervical vertebra of another individual, from 

 a different locality, in which the centrum is relatively shorter than in the two fore- 



