CRETACEOUS PLESIOSAURS. 



219 



centrum which the vertebrae undergo as they pass from the region of the neck to that 

 of the back, without corresponding alteration in the length of the centrum. 



The following are dimensions of the most perfect specimens of these vertebrae : 



The above dimensions show that whilst the centrums retain the length of two 

 inches in the middle and towards the posterior parts of the long neck, they become 

 shortened in the penultimate and last cervicals to the length of the smaller vertebra 

 towards the anterior part of the neck ; the diiference, however, is but' slight, and 

 whilst an almost uniform length is retained, the vertebral centrums augment in height, 

 and still more in breadth, as they approach the region of the back. 



With the increased breadth of the centrum, there is a concomitant increase in that 

 of the rough depressions (PI. 29, figs. 3 and 4, np) for the articulation of the neur- 

 apophyses, and, at the same time, the bases of these vertebral elements become wider 

 apart, and the breadth of the surface (ib. w n) supporting the neural axis, increases. 

 This smooth surface which occupies the middle of the upper part of the centrum is 

 contracted in the middle by the approximated neurapophysial pits, where there is on 

 each side the orifice of the canal for the vertebral vein or sinus which traverses the 

 centrum vertically. The lower openings of these canals are shown in PI. 29, figures 

 2 and 5, and their whole course is displayed in the fractured vertebra rrepresented in 

 fig. 6, cc'. 



The costal pits in the greater proportion of the cervical vertebrae present the form of 

 a full transverse ellipse, as in PI. 28, fig. 1, and are situated below the neurapophysial pit 

 at a distance about equal to their own vertical diameter. They are nearer the posterior 

 than the anterior surface of the vertebra, and thus difi'er in position as in shape from 

 the costal surface in PI. 27, fig. 3, j,i. As the cervicals approach the dorsal region 

 the costal pit increases in vertical extent, assumes a circular form, and, as in all 

 Plesiosauri, begins to rise towards the neurapophyses. The commencement of this 



