2 FOSSIL REPTILIA OF THE 



Plesiosaukus 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 flat- 

 ness, 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 surfaces (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 neurapophysial ( 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 all the bones derived from 

 the stratum of Cambridgeshire phosphatic Green-sand being more or less rubbed or 

 worn, either in the original imbedding, or subsequently by the mechanical appliances 

 by which the phosphatic nodules are extracted. 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 

 vertebrse 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, 1\ 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- 



