﻿CRETACEOUS FORMATIONS. 



23 



phosphatic-nodule works at Haslingfield, about four miles from the town of 

 Cambridge. 



In a collection of Upper Green-sand fossils from the vicinity of that town, 

 lately purchased by the British Museum, there is the centrum of a dorsal vertebra 

 of corresponding dimensions. It presents the usual characters of the Plesio- 

 sauroids ; the articular ends are very slightly concave, with a moderate promi- 

 nence in the middle, of a subcircular form, about the size of a crown-piece. 

 The sides are gently concave lengthwise ; the under surface is so in a less 

 degree ; this non-articular surface is smooth at the middle part, with longitudinal, 

 irregularly wavy ridges and grooves for an inch at the margin, which are well 

 defined ; this roughness indicates the attachment of the fibres of the capsular 

 ligament. The fore-and-aft diameter of the centrum is less at the summit than 

 at the base ; here it measures 4 inches 6 lines ; along the neural canal it is 4 

 inches ; the smooth tract caused by the impress of this canal is 6 lines across the 

 narrowest part, and 2 inches across the widest end. The neurapophysial pits are 

 shallow, with a rugged surface 3 inches 6 lines long by 1 inch 9 lines in diameter ; 

 the small part of the upper surface of the centrum not covered by the neurapo- 

 physis is at the end where the neural canal is widest, and which is most probably 

 the hinder end ; there are two venous foramina on one side and three on the 

 other side of the middle of the lower surface of the centrum. The breadth of 

 the articular surface is 6 inches 3 lines ; its depth, or vertical extent, the same. 



The same conformity, in regard to their proportional size, characterises the 

 teeth of Polyptychodon and the associated large Plesiosauroid vertebras from 

 Kursk. I am indebted to the able engineer and zealous palaeontologist, Colonel 

 Kiprianoff, for the opportunity of examining the specimens discovered by him in 

 that locality. 



The centrum of one of these vertebras belonging to the dorsal region, from the 

 Neocomian formations at Kursk, measures 4 inches in length and 5 inches 4 lines 

 in breadth ; the terminal articular surfaces are flat ; between them the lower surface 

 of the centrum is straight, but at the sides it is gently concave ; there are two 

 venous foramina, 2 lines apart, at the middle of the under surface of the 

 centrum. 



Portions of ribs from the Upper Green-sand of Cambridgeshire agree in 

 texture, and correspond in proportional size, with the cervical and dorsal vertebral 

 bodies with which they were associated. I have selected one of these fragments 

 for representation in Tab. V, fig. 3, because it shows a well-marked ridge ( s ) on 

 one side, a character I have not seen in the ribs of true Plesiosauri ; and these 

 portions of ribs, of probably Polyptychodon, present a less rounded transverse 

 section. 



