ON BRITISH FOSSIL. REPTILES. 63 



only be supported and wielded by a neck as short and strong as in the Ceta- 

 ceous inhabitants of the sea. 



The cervical vertebrae, as they recede from the head, increase in breadth 

 and depth, but retain the same length, as they do throughout the spine in 

 most Saurians, whatever may be their other dimensions. But in the dorsal 

 region of the spine of the Pliosaur, the vertebrae acquire a great increase of 

 length, and there assume the ordinary proportions of Plesiosaurian vertebras : 

 for example, the first dorsal vertebra of the Market-Raisin specimen, which 

 is four inches three lines in breadth, and four inches in depth, measures 

 nearly three inches in length. The posterior dorsal vertebrae slightly in- 

 crease in depth, and with the same transverse diameter they present a length 

 of 3 inches 2 lines. The height of one of these vertebrae, including the 

 spinous process, is 11 inches. These proportions are retained at least to the 

 base of the tail. A vertebra from this part, obtained from St. Giles's gravel- 

 pit, near Oxford, and probably washed out of the Kimmeridge clay, measures 

 in length 3 inches ; in the breadth of the body, 4 inches 9 lines ; in the depth 

 of the same, 4 inches 4 lines. 



In the extreme difference which the vertebrae of the neck and those of the 

 rest of the trunk present in regard to their length, the Pliosaurus forms a 

 remarkable exception to Saurians in general ; for in the true Enaliosaurs, in 

 Crocodiles, in Lizards, whatever other modifications the vertebrae may undergo, 

 or however much they may be expanded in breadth or depth, they maintain 

 great constancy in the length or antero-posterior diameter of the body. The 

 Pterodactyles, or flying-lizards, offer another exception to this rule, and the 

 cervical region is here likewise the seat of the variation ; but whereas in the 

 Pliosaur the cervical vertebrae are remarkable for their shortness ; in the Pte- 

 rodactyle they differ from the other vertebrae in their extreme length. 



The general structure of the vertebrae of the Pliosaur corresponds closely 

 with that of the Plesiosaur. The osseous texture is compact at the circum- 

 ference of the vertebras, and coarsely, but uniformly, cellular in the rest of 

 the bone. The neurapophyses do not become anchylosed to the centrum, 

 nor the ribs to the costal processes. The articular surfaces at each end of the 

 centrum are flat in the cervical, very slightly concave in the dorsal, rather 

 more concave in the caudal vertebrae. The cervical ribs, judging from their 

 articulation with the centrum, must have been unusually strong. The rib on 

 each side of the vertebra was supported on two transverse processes, slightly 

 raised beyond the level of the centrum, occupying two-thirds of its antero- 

 posterior extent, and divided by a deep and well-marked linear fissure. In 

 the anterior cervical vertebrae above described, with a length of 1 inch 9 lines, 

 and a height of centrum of 3 inches 3 lines, the antero-posterior diameter of the 

 constant surfaces was 1 inch 2 lines ; their united vertical diameter, 2 inches 

 2 lines. They occupy a space nearly equi-distant from the upper and lower 

 surfaces of the centrum. At the base of the neck they begin to rise, as in the 

 Plesiosaur, upon the neurapophysis, and are supported, in the dorsal region, 

 upon a single stout transverse process. This is subdepressed, with an oval 

 transverse section, which is rather sharp at the anterior margin. The spinous 

 process of the dorsal vertebrae is nearly straight, compressed laterally ; its an- 

 tero-posterior diameter was 2 inches 8 lines ; in a vertebra, measuring in the 

 same diameter 3 inches 2 lines, its height from the base of the neurapophysis 

 was 7 inches. The sides of the centrum are rather rugous near the articular 

 ends, elsewhere smooth and concave, especially in the dorsal vertebrse. The 

 lower surface has the two vascular perforations in the cervical regions ; the 

 vertebrse become slightly contracted towards this part in the dorsal region. 

 In the caudal vertebrae the costal process is single, vertically elliptical, and 



