LIASSIC PLESIOSAURS. 7 



ward, looks downward, outward, and a little backward, the process being slightly 

 inclined that way. The margin of the articular end of the centrum is better defined 

 than in the neck; about a line's breadth is, as it were, shaved off; the rest of the sur- 

 face (fig. 2, c) is very slightly concave, sometimes undulated, always nearly flat, and 

 with a small central depression, or a tendency there to a tubercle. The length 

 of tlie dorsal region in the skeleton (Tab. I) 8 feet 9 inches long, is 2 feet G inches ; 

 the number of dorsal vertebrae is twenty-one. 



Sacrum. — Two vertebrae (ib., s) succeeding the dorsals are distinguishable, through 

 the greater thickness and straightness of their short pleurapophyses, as sacral ; these 

 elements abut against, or afford ligamentous union to, the iliac bones. 



Caudal series. — The caudal vertebrae (Tab. I, c d, Tab. IV, figs. 3 — 9) are shorter in 

 proportion to their breadth than the others ; the centrum approaches to a cubical figure, 

 the under surface (figs. 4 and 7) becoming broad and flattened ; and the contour of the 

 terminal articular surfaces shows a similar tendency to flattening, giving a transversely 

 extended quadrate figure, with the angles rounded off (figs. 5 and 8) ; the margin is 

 thicker, more rounded off, less defined than in the dorsal vertebrae. The articular surface 

 itself is more concave than in the antecedent regions of the backbone, and becomes 

 deeper in the terminal subcompressed vertebrae (fig. 9) ; the movements of the tail 

 in swimming having been helped here by a greater amount of yielding intervertebral 

 substance, approaching in the same degree to the condition of the spine in fishes. 

 The costal surface (figs. 3, 6, pi) is elliptic, with the long axis subvertical, the margin 

 prominent, the cavity simple and rough for the ligamentous attachment of the ril:)let ; it 

 is situated on the upper half of the centrum close to the neurapophysis, the outer end 

 of the base of which contributes to the upper part of -the margin in the anterior 

 caudals (fig. 5, d). 



The pleurapophyses in this region (Tab. IV, 5, pi) do not expand terminally, 

 as in the neck ; they are short, thick, and straight, simulating transverse processes ; 

 their non-confluence with the centrum exemplifies the minor vigour of vital co-ossifying 

 influences in terminal parts. 



The heemapopliysial surfaces (fig. 3 A') impress the inferior angles of the 

 posterior surface of the centrum ; occasionally, where a haemapophysis has become 

 anchylosed and broken off, its adherent base gives the appearance of a process from 

 that part.;t)f the centrum (ib., figs. 6, 7, S, /*')• The venous foramina are at the lower 

 part of the sides of the centrum. The neural arch (figs. 3, 5, «) rapidly diminishes in 

 size and in the length of the neural spine, ns. The zygapophysial surfaces become more 

 vertical, the anterior, z, looking inward ; the posterior zygapophyses, z', are the first to 

 disappear. Tlie haemapophyses (fig. 5, h) are free, and were ligamentously connected 

 with the centrum above and with each other below, circumscribing there the haemal 

 canal. The proximal surface is expanded, with a subtriangular facet cut obliquely at 

 the anterior part for articulation with its own surface, and with a smaller, less definite 



