66 FOSSIL REPTILIA OF THE 



change in form and position of the costal pit, pi, is shown in T. XX, fig. 3, and its 

 borders are here seen to be rather prominent. In none of the vertebrae has the costal 

 pit presented the groove which, in most Plesiosauri, crosses it in the axis of the vertebra 

 and divides it into two subequal parts. The articular ends of the centrum are shghtly 

 concave and are impressed by a circular pit at the centre ; the peripheral margin is 

 rounded off ; it appears in the side view of the vertebra, fig. 3, T. XX, but not to 

 such an extent as in Plesiosaurus Bernardi, T. XVIII, fig. 2. The lower apertures of 

 the venous canals are closely approximated in all the cervicals except the most pos- 

 terior ones, in which the canals diverge, as they descend, with a proportionate breadth 

 between their lower outlets, c' c', fig. 2, T. XXI. They are divided by a narrow ridge, 

 as in fig. 5, in the ordinary cervical vertebrae, and are not situated in fossae, as in the 

 Plesiosaurus Bernardi, T. XVIII, fig. 4. 



In the vertebra which I take to be the penultimate or antipenultimate cervical, the 

 upper half of the costal surface has passed upon the base of the neurapophysis, and, 

 from what remains upon the centrum, as at pi, fig. 5, T. XX, we may see that the 

 surface has undergone a further change of form, and has exchanged the circular (as 

 in fig. 3) for a vertically elliptical or oval figure. 



In the centrum of the last cervical vertebra figured in T. XXI, figs. 1 , 2, 3, the 

 last trace of the costal surface is shown at pi, fig. 1 . A.nd I may here remark, that, as 

 there is no definite natural distinction between the cervical and dorsal regions of the 

 Plesiosaurus, the vertebrae in both supporting ribs, and the transition in the size, 

 shape, and position of these being more gradual than in the Crocodiles, I have selected 

 the arbitrary character of the impression of the costal articular surface, or any part of 

 it, upon the centrum, as the character of the cervical vertebrae in Xhe Plesiosaurus, and 

 I count that to be the first dorsal in which the costal surface has wholly ascended upon 

 the neurapophysis. 



In T. XXI, fig. 7, one of the caudal vertebrae is figured showing the longitudinal 

 channel, at the middle of the under surface, bounded by the ridges which terminate on 

 the articular surfaces for the haemapophyses : those surfaces are here worn away. The 

 neurapophyses have coalesced with the centrum ; and the ribs have also coalesced, 

 forming the ' transverse processes' of this caudal vertebra. 



Plesiosauroid Paddle. Table XVII. 



The block of Chalk from the pit at Burham, in Kent, figured in Tab. XVII, 

 includes parts of four digits of the same foot, the phalanges of which had the opposed 

 ends flattened, and joined together by ligament, the whole forming part of the bony 

 framework of a large fin, most resembling that of the Plesiosaurus. This fine specimen 

 forms part of the rich Collection of Chalk-fossils belonging to Mrs. Smith, of Tonbridge 



