412 BRITISH FOSSIL REPTILES. 



of the base of the neurapophysis is 2 inches 2 lines ; and it does not begin 

 so close to the anterior part of the centrum as in the dorsal vertebra. There 

 are no posterior zygapophyses. The upper and lower portions of the side of the 

 centrum are more distinctly separated by the comparative projection of the middle 

 part, which gives the obscurely hexagonal form to these vertebrae. The inferior 

 parts are most concave, and converge to form the sides of the longitudinal sulcus, 

 to which the inferior surface of the centrum is reduced at this part of the tail. 

 It is plain, from these modifications of the vertebrae, that the tail must here 

 have presented the compressed CrocodiUan type ; and it is satisfactory to have 

 these indications of the Saurian affinities of the present gigantic fossil, in con- 

 sequence of the very close approximation of the larger vertebrae to the Cetaceous type. 

 The vertical extent of the osseous basis of the tail was here augmented by strong 

 haemapophyses, which have left more prominent articular facets on the under part 

 of the centrum than in the larger anterior caudal vertebrae : these facets, instead 

 of being in pairs, as in the anterior caudals, approximate, and become confluent 

 in the vertebrae of between 3 and 4 inches in breadth. 



Occasionally the haemal arch is found anchylosed to the posterior of these so 

 confluent haemapophysial surfaces, as in the posterior caudal vertebra figured in 

 PI. 31, figs. 3 and 4. 



A vertical section, through the middle of a dorsal vertebra, from that part of 

 the back where the rib has ascended to articulate wholly with the diapophysis, 

 well displays this characteristic modification of the articular parts of the centrum, 

 in Cetiosa/trus (PI. 35, fig. 2). The same section shows the closer cancellous 

 texture of the centrum near those articular ends ; the more open texture, with a 

 general tendency to a longitudinal course of the cancelli, in the middle ; and the 

 still more open and irregularly disposed cancellous structure at the base and back 

 part of the neural spine. 



From the foregoing data it may be inferred that there existed, at the period of the 

 deposition of the Wealden, a Saurian reptile of dimensions at least equalling those of 

 the Iguanodon, but with modifications of the vertebral column, from the middle of 

 the back to the tail, departing from the Dinosaurian and approaching to the 

 Crocodilian type. If, as is very probable, the cervical and anterior dorsal vertebrae 

 above described (pp. 400 — 404, and provisionally referred to Streptospondylus), belong 

 to the same reptile as the succeeding vertebrae, here referred to Cetiosaurus, we 

 should then have a gigantic Crocodilian of the peculiar transitional type, as between 

 that order and the Dinosaurian, which is manifested by the second " Honfleur Gavial" 

 of Cuvier; i. e., with convexo-concave vertebrfe at the fore part of the trunk, 

 graduating into plano-subconcave vertebrae, with elevated and somewhat complex 

 neural arches, at the middle and back part of the trunk, and with vertebrae sub- 

 concave at both ends, in the tail. 



