﻿FOSSIL REPTILIA OF THE 



ratio as the vertebra? enlarge and recede in position; so that the length of the centrum 

 may be one sixth less than the breadth at the middle (Tab. Ill, figs. 4, 5, 6) and hind 

 part of the neck. There are, however, varieties in this respect, and the equality of 

 length to breadth of centrum is maintained through a greater extent of the neck in 

 some specimens than in others. The vertical diameter of the middle of the terminal 

 articular surface (ib., fig. 6 c ) is less by nearly one fourth than the breadth of the same. 

 The sides of the centrum are longitudinally concave (ib., fig. 5), as is also the under part 

 (ib., fig. 4), but in a minor degree in the middle and posterior than in the small an- 

 terior cervicals (ib., fig. 1, c). 



The costal surface is at the lower part of the side of the centrum ; it is narrow 

 vertically, in proportion to its length in the anterior cervicals, but gains in vertical 

 extent without being elongated in the same degree, and consequently occupies a 

 larger corresponding extent in the middle cervicals (ib., fig. 4, pi),* in which the costal 

 surface exceeds one half the length of the centrum ; it is divided by a longitudinal 

 cleft, and is situated a little nearer the posterior than the anterior surface of the 

 centrum. In the third to the seventh cervicals (ib., fig. 1, cj) the costal surface is 

 separated by a tract exceeding its own vertical diameter from the neurapophysial 

 surface (ib., fig. 1,«); in the succeeding cervicals the intervening tract equals the 

 costal surface (ib., fig. 4, c ) ; and the interval is never less, and is sometimes more, in 

 the cervical vertebra) to near the base of the neck. The terminal articular surface 

 (ib., fig. 6) is moderately convex at its periphery and very gently concave in the rest of 

 its extent, with a small central, often transversely linear, impression in the centre. f 



The free surface of the centrum is finely rugose in the smaller anterior cervicals, 

 and is not smooth in any of the others ; towards the articular ends the roughness is 

 more marked, by irregular narrow risings and groovings, which become more longitu- 

 dinal in direction in the succeeding cervicals (ib., figs. 4, 5). The under surface (fig. 5) is 

 concave transversely from the costal pit (pi) to the two venous openings, and is convex 

 between those openings, which divide the surface pretty equally into three parts, 

 Lengthwise, as already stated, the under surface is gently concave. The neurapo- 

 physial surface is less angular than in some other species, the lower angle being 

 rounded off, making the lower border approach to a curve. Anchylosis of the neural 

 arch with the centrum seems to have been complete in the anterior cervicals of the 

 specimen figured in Tab. Ill, fig. 1 ; and I have not yet seen a cervical centrum of 

 the present species from which the neural arch had become detached, save by fracture. 

 The zygapophyses are proportionally large ; the anterior ones (ib., z, z ) extend forward, 

 in the anterior vertebrae almost immediately above the centrum, overhanging the 



* Here obscured by the confluent base of the rib. 



t " The concavity again slightly swelling in a contrasted curve near the middle of the circular area," 

 (Conybeare's first Memoir, p. 582, April, 1821) is the character of the terminal articular surface in the 

 Plesiosaurus arcuafus, from the Lias in the neighbourhood of Bristol. 



