98 REPORT— 1841. 



the surface *. The sides of the centrum at the upper half are concave both 

 lengthwise and vertically, forming a wide depression below the transverse 

 process ; the middle part of the side begins to stand out and divide the upper 

 from the lower lateral concavity, which character, being more strongly deve- 

 loped in the hinder caudal vertebrae, gives the rhomboidal or hexagonal formf. 

 The lower half of the side of the centrum is less concave than in the dorsal 

 vertebrae. The broad inferior surface is also less concave antero-posteriorly 

 than in the dorsal vertebrae, and is nearly flat transversely : it gradually con- 

 tracts, in the transverse direction, in the posterior caudals, so as to take on the 

 form of a longitudinal sulcus. The two anterior haemapophysial surfaces are 

 separated from each other by an interval of two inches ; the two posterior 

 surfaces, which are larger than the anterior ones, are similarly distinct. 



In the Iguanodon the haemapophysial surfaces are confluent on both the 

 anterior and posterior parts of the under surface of the centrum, and the che- 

 vron bones accordingly present modifications by which they may, when de- 

 tached, be distinguished from those of the Cetiosaurus, 



The transverse processes have descended, as usual, from the summit to the 

 base of the neural arch in the present anterior caudal vertebrae. They are 

 short, compressed vertically, diminishing, and as if slightly twisted, so that the 

 upper margin is turned forwards, at their extremity. The vertical diameter 

 of the base of the transverse process in the largest of the present caudal ver- 

 tebrae is three inches ; its anterior-posterior diameter one inch six lines ; its 

 length two inches seven lines : the extremity terminates obtusely. The upper 

 ridge-like termination of the transverse process is continued to the base of 

 the anterior oblique processes. These processes are alone developed in the 

 present vertebree, the posterior articular surfaces being impressed upon the 

 sides of the posterior part of the base of the spine. The anterior oblique 

 processes project almost horizontally forwards, diminishing, chiefly in vertical 

 diameter, to an obtuse apex ; convex externally, flattened internally by the 

 oblong articular surface, and separated by a fissure nearly one inch wide : 

 the length of these processes, from the bottom of the intervening fissure in 

 the second of the four caudals, where they are most entire, is two inches. 

 When the vertebrae are placed in juxtaposition, these processes reach beyond 

 the middle of the vertebrae next in front, and pinch, as it were, the back part 

 of the base of the spine so as to impress upon it the surfaces representing the 

 posterior articular processes. These processes are well developed, on the 

 contrary, in the corresponding vertebraj of the Iguanodon, and overhang the 

 posterior surface of the body of the vertebra to which they belong. The spi- 

 nous process, which appears to be nearly perfect in the second caudal, is short, 

 strong, and truncated at the summit. Its height from the anterior oblique 

 processes is four inches : the total height of the vertebra is thirteen inches. 

 The antero-posterior diameter of the side of the neural arch is two inches. 

 The spinal canal is wider in these caudal than in the dorsal vertebrje, indica- 

 ting the greater muscularity of the part deriving its nervous po-wer from the 

 corresponding part of the spinal chord : its transverse diameter is one inch ten 

 lines ; its vertical diameter is two inches. The neural arch is, as usual in the 

 present genus, anchylosed to the anterior part of the upper surface of the cen- 

 trum : one inch and a half of this surface is left free behind the attachment 



* The same modification of the articular extremities occurs in the caudal region of the ver- 

 tebral column of the Plesiosaurus. See ' Report on Brit. Foss. Reptiles,' iiart i. Trans. Brit. 

 Assoc. 1839, p. 58. 



t It is one of these posterior caudals of the Cetiosaurus, whicli is figured as the type of 

 the " second vertebral system " in the ' Geology of the South-east of England/ p. 296, fig. 2. 



