2 FOSSIL REPTILIA OF THE 



Plesiosaurus neocomiensis, Cpche. ' Description des Fossiles du Terrain Cre- 



tace des Environs de Sainte-Croix,' 4to, 

 1858—1860, par N. J. Pictet and G. 

 Campiche, p. 12, pi. vi. 



The following are descriptions of other species of Cretaceous Plesiosauri, with 

 additional illustrations of already indicated species : 



Plesiosaurus planus, Owen. Vertebral Centrums, Tab. I, II, and III. 



The cervical centrum selected for the figures 1 — 4, Tab. I, gives the characters 

 afforded by this instructive part of the vertebral column of a Plesiosaurus. The 

 flatness, both of the under (fig. 4) and of the terminal articular surfaces (fig. 2), 

 suggested the name distinguishing the species, or at least the vertebrae by which 

 alone this cretaceous Plesiosaur has hitherto been exemplified. The costal sur- 

 faces (Tab. I, figs. 1, 2, and 4, pi) are of a narrow, oblong figure, formed, as it were, 

 by truncation of the lower angles of the triangular centrum, of which the apex has 

 been more broadly removed by the sections, leaving the neural (ib., n) and neurapo- 

 physial {np) surfaces above. If the borders of the costal surface have projected 

 with a sharper definition, they have been abraded, as, indeed, is most probable ; 

 almost ail the bones derived from the stratum of Cambridgeshire phosphatic Green- 

 sand being more or less rubbed or worn, either in the original imbedding, or sub- 

 sequently by the mechanical appliances by which the phosphatic nodules are ex- 

 tracted. I have selected the centrum which has been least subject to this attrition, 

 from a large series of the present species. Subsequent observers, who may have 

 been favoured with entire and unworn fossil vertebrae with the main features and 

 proportions of the Plesiosaurus planus, will make allowance for the circumstances 

 in which the materials for reconstructing that species first came to hand. 



What may be more certainly predicated of the costal surface is the absence of 

 depth and of linear horizontal bisection, both which characters are present in the 

 cervicals of some other Plesiosauri. The distance between the costal and neura- 

 pophysial surfaces is nearly three times that of the vertical diameter of the former, 

 and the intervening non-articular surface is smooth, and also plane or flat, sloping 

 upward towards the neurapophysial border, and showing no sinking or concavity 

 in the longitudinal direction. The neural surface, 2^ lines in breadth at the 

 narrowest part, slightly expands towards the posterior surface of the centrum. 

 The neurapophysial surfaces are coextensive with the long or fore-and-aft diameter 

 of the centrum, and of nearly equal breadth anteriorly ; they are smooth and very 

 shallow, with a slightly defined, thin border, which is undulated outwardly, descend- 



