214 BRITISH FOSSIL REPTILES. 



dant ; and I shall here limit myself to quoting the brief but graphic definition of 

 it which Dr. Buckland has given in his interesting and instructive ' Bridgewater 

 Treatise :' — "To the head of a Lizard it united the teeth of a Crocodile ; a neck of 

 enormous length, resembling the body of a Serpent ; a trunk and tail having the 

 proportions of an ordinary quadruped ; the ribs of a Chameleon ; and the paddles of a 

 Whale. Such are the strange combinations of form and structure in the Plesiosaurus," 

 (p. 102.) I may add, that of all existing Reptiles the Chelonians make the nearest 

 approach to the present remarkable extinct genus in the length and flexibility of the 

 neck, in the size of the true body of the atlas, which resumes its normal relations 

 with the neural arch of that vertebra in Cheli/s and Chelodina, as in Plesiosaurus ; 

 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 (hsemapophyses and their spines), which form a kind 

 of interwoven flexible " plastron " beneath the abdomen. 



Plesiosaurus Bernardi, Oiven. ' Enahosauria,' Plate 26. 



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 PI. 26, 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. 



Antero-posterior diameter of centrum 

 Transverse diameter 

 Vertical diameter 



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 



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



t Ibid., p. 193. This specimen was kindly transmitted to me by J. Wickham Flower, Esq. 



