60 FOSSIL REPTILIA OF THE 



in the natatory form of the extremities as exemplified in the paddles of the Turtle, 

 which besides being four in number, come much nearer those of the Plesiosaurus in 

 structure than the paddles of the Whale do, and in the great expanse of the ischium 

 and pubis : whilst the Plesiosaurs exhibit, next to the Turtles, the greatest deve- 

 lopment of the abdominal ribs (haemapophyses and their spines), which form a kind 

 of interwoven flexible " plastron " beneath the abdomen. 



Plesiosaurus Bernardi, Owen. Tab. XVIII. 



Dixon's ' Geology and Fossils of the Tertiary and Cretaceous Formations of Sussex,' p. 396. 



In my ' Report on British Fossil Reptiles,' one species of Plesiosaurus, viz. Plesio- 

 saurus pachyomus, was defined from remains discovered in the green-sand division of the 

 Cretaceous series ;* and the existence of the genus Plesiosaurus, at the period of the 

 deposition of the latest member of that series, was inferred from the discovery of the 

 femur of a large species in the chalk which forms the well-known " Shakespeare's 

 Cliff' near Dover.f 



This indication has been since confirmed by the discovery not only of the teeth 

 above described, but of vertebrse of the Plesiosaurus in the same formation ; and the 

 cervical vertebra figured in T. XVIII, which was obtained from the Upper Chalk at 

 Houghton, near Arundel, Sussex, indicates a species allied to the Plesiosaurus 

 pachyomus from the green-sand of Cambridge. 



The following are the dimensions of the vertebra from Houghton, and of the most 

 perfect of those of the above-cited species from the green-sand. 



PI. pachyomus. PL Bernardi. 



Inches. Lines. Inches. Lines. 



Antero-posterior diameter of centrum ... 2 1 9 



Transverse diameter ...... 2 9 3 



Vertical diameter 2 6 2 



The breadth of the centrum is proportionally greater in the vertebra from the 

 chalk, which further differs from that from the green-sand in the lower position, and 

 the anchylosis of the pleurapophyses, pi (hatchet-bones or cervical ribs) ; which, if they 

 presented the characteristic expansion of their extremities, must have supported the 

 hatched-shaped head on an unusually long body or pedicle. The articular surfaces of 

 the centrum are more concave than in most Plesiosauri, and deepen to a central pit, 

 in which they resemble those of the Plesiosaurus pachyomus ; but the circumference of 

 the articular surface is more extensively rounded or bevelled off, so that its convexity 

 is seen, as at ca, cp, upon a side view of the vertebra, fig. 3, Tab. XVIII. 



* Report on, British Fossil Reptiles, Trans. Brit. Association (1839), p. 74. 



■f Ibid., p. 193. This specimen \ras kindly transmitted to me by J. Wickham Flower, Esq. 



