6 FOSSIL REPTILIA OF THE 



pachyomus (1. c, Tab. XX, fig. \,pi). The under surface of the centrum is flat 

 from before backward, and describes a gentle uniform convexity from one costal 

 pit to the other. 



The vertebral centrum, Tab. II, figs. 3, 4, 5, is from the base of the neck, 

 and from a larger individual. The bases of the neurapophysial pits (fig. 4, 

 np) have not been coextended with the increased length of the centrum, and the 

 apex contracts more quickly, and is extended to the upper division of the costal 

 pit. The breadth of the neural surface (ib., n ) is the same as in the more anterior 

 cervical centrum (fig. 2) ; but the orifices of the venous canals are more con- 

 spicuous. Only a small part of the costal pit (ib., pi ) now marks the centrum ; 

 it projects from the side of that element, nearer its posterior surface. The 

 articular surfaces of the centrum (ib., fig. 3, c) are nearly flat, and slightly undu- 

 lating, without a central pit. The lower orifices of the venous canal are about 

 two lines apart. 



The centrum, Tab. II, figs. 6 — 9, is from the base of the neck of another and 

 larger individual of the Plesiosaurus planus, and, with a moderate increase of all 

 its dimensions, shows least that of breadth. The articular surface of the centrum 

 (fig. 6, c ) has a shallow depression at its middle part, occupying about half the breadth 

 of the surface ; it is flat at the circumference, and its margin, though obtuse, is 

 narrow and well defined. The narrow outer part of the neurapophysial tract (ib., 

 fig. 9, np) has a well-defined raised border, terminating in the major part of the 

 costal surface, the lower half of which is much reduced in size; the interspace is 

 occupied by a small mass of matrix. The under surface shows a slight concavity 

 from before backward. The non-articular surface of the centrum is almost 

 smooth. 



A similar and closely succeeding vertebral centrum of the same species of 

 Plesiosaurus is figured in Tab. Ill, figs. 5 and 6. It is more mutilated, and a 

 portion of a rib is cemented to the neural surface (fig. 6). The costal surface has 

 risen wholly upon the neurapophysis {np), the base of which adheres to the 

 centrum, and projects outward as a costal diapophysis [d). This centrum is 

 from the fore part of the dorsal region. 



The cervical centrum (Tab. Ill, figs. 1 — 4) appears to have come from the basal 

 third of the neck, perhaps from the beginning of that part, in which the contour 

 of the articular surface, expanding towards the lower part, takes on, as in the 

 antecedent cervicals (Tab. I, fig. 2), something of a triangular form ; here, however, 

 the shape of the neurapophysial surfaces {np) is of a more regular triangular form 

 (compared with fig. 2, Tab. II) and they are connected by a narrow, slightly elevated 

 tract with the costal pit { p i). This articular surface begins to diminish in antero- 

 posterior extent, indicating a corresponding change in the shape of the shaft of 

 the costal rib ; the terminal articular surface of the centrum has a slight central 



