410 BRITISH FOSSIL REPTILES. 



the centrum is less concave than in the dorsal vertebrpe. The broad inferior surface 

 is also less concave antero-posteriorly than in the dorsal vertebrce, and is nearly flat 

 transversely; it gradually contracts, in the transverse direction, in the posterior 

 caudals, so as to take on the form of a longitudinal sulcus. The two anterior heem- 

 apophysial 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 anterior as well as posterior caudal vertebrae of the Iguanodon the 

 hajmapophysial surfaces are confluent on both the anterior and posterior parts of 

 the under surface of the centrum, and the chevron bones accordingly present 

 modifications by which they may, when detached, be distinguished from those of 

 the Cetiosaiirus. There was, however, as will be presently shown, another gigantic 

 Saurian of the Wealden period, distinct from the Cetiosaurus and Iguanodon, but 

 resembling the latter in the single hsemapophysial facet (11. 37). 



The diapophyses, in the caudal vertebrae of Cetiosaurus (Pis. 23 and 36), have 

 descended, as usual, from the summit to the base of the neural arch in the 

 anterior caudal vertebrae. They are short, compressed vertically, diminishing, and 

 as if slightly twisted, so that the upper margin is turned forward, at their extremity. 

 The vertical diameter of the base of the transverse process in the largest of the 

 present caudal vertebrae is 3 inches ; its antero-posterior diameter is 1 inch 6 lines ; 

 its length is 2 inches 7 lines : the extremity terminates obtusely. The upper ridge-like 

 termination of the transverse process is continued to the base of the anterior zygapo- 

 physis. These processes (ib., z) are alone developed, as such, in the present vertebrae ; 

 the posterior articular surfaces (PI. 36, z) being impressed upon the sides of the pos- 

 terior part of the base of the neural spine. The anterior zygapophyses project almost 

 horizontally forward, diminishing, chiefly in vertical diameter, to an obtuse apex ; 

 convex externally, flattened internally by the oblong articular surface, and separated 

 by a fissure nearly 1 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 2 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, ilie back 

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

 posterior zygapophyses. These processes are well developed, on the contrary, in the 

 corresponding vertebrae of the Iguanodon, and overhang the posterior surface of the 

 body of the vertebra to which they belong. The spinous 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 4 inches : the total height of the 

 vertebra is 13 inches. The antero-posterior diameter of the side of the neural arch 

 is 2 inches. The spinal canal is wider in these caudal than in the dorsal vertebrae, 

 indicating the greater muscularity of the part deriving its nervous power from 

 the corresponding part of the spinal cord : its transverse diameter is 1 inch 



