CRETACEOUS FORMATIONS. 15 



by the proprietor, William Harding Bensted, Esq., to whom the earlier discovery 

 of remains of an Iguanodon in the same locality and formation, is due.* 



My first knowledge of these remains was obtained from plaster casts of the two 

 most complete vertebrae which were transmitted to me by Mr. Bensted for deter- 

 mination of the species in 1853, which casts were afterwards presented by Mr. 

 Bensted to the Museum of the Royal College of Surgeons/)- The original of these 

 casts, with the other portions of the skeleton discovered by Mr. Binsted, have since 

 been purchased by the Trustees of the British Museum. 



From the Plesiosaurus pachyomus, Owen, of the Upper Green-sand of Cambridge- 

 shire, the present species differs in the greater relative length and breadth of the 

 centrum in proportion to its height, in the smaller relative size of the costal surface, 

 its greater prominence, and inferior position upon the side of the centrum, where 

 it is supported by a low parapophysis (compare Tab. VII with Tab. XX, torn, cit., 

 Monogr. Cretaceous Reptiles). In that plate are represented the centrums of 

 three cervical vertebrae of the Plesiosaurus pachyomus ; one (fig. 1) giving the charac- 

 ters of the ordinary or more numerous cervicals ; a second (fig. 2) showing the 

 commencement of the rise of the costal surface, and the development of the 

 vertical ridge connecting it with the neurapophysial surface ; a third (fig. 3) showing 

 the junction of the two articular surfaces indicative of the passage of part of the 

 head of the pleurapophysis upon the base of the neurapophysis. 



The following are dimensions'of an ordinary cervical centrum of the two species : 



Plesiosaurus lalispinus Plesiosaurus pachyomus. 

 In. lines. In. lines. 



Length '. . 2 8 . 1 11 . 



Breadth 30 . 23 



Height 26 . 23 



Fore-and-aft diameter of the costal surface ..10 . 14 



The borders of the terminal articular surface are thinner and more defined in 

 Plesiosaurus lalispinus than in Plesiosaurus pachyomus. The costal surface (Tab. VII, 

 fig. 1, pi) is longitudinally coextensive, in Plesiosaurus latispinus, with little 

 more than one third of the fore-and-aft extent of the centrum. In Plesiosaurus 

 pachyomus it is coextensive with two thirds of the same extent. In Plesiosaurus 

 latispinus it is situated so low down as, in a direct side view, to mask part of the in- 

 ferior contour of the centrum. In Plesiosaurus pachyomus it allows the whole of the 

 lower contour to be seen in the same side view. In Plesiosaurus latispinus more than the 

 vertical diameter of the costal surface, by one fifth or one sixth, intervenes between it 



* See ' Monograph on the Fossil Keptilia of the Cretaceous Formations,' volume of the Palfeonto- 

 graphical Society for 1851, p. 105. 



f See ' Descriptive Catalogue of the Fossil Reptilia and Pisces,' 4to, p. 63, No. 251, 



