WEALDEN FORMATIONS. 15 



In. Lines. 

 Anteroposterior diameter of anterior costal surface . . .17 



Antero-posterior diameter of posterior one . . . 10 



Breadth of spinal canal . . . . .15 



Breadth of canal of sacral nerve . . . . 4 



From its separated condition, the body of the sacral vertebra here described must 

 have belonged to a young Dinosaur of a size far exceeding that of the Hylaosaurus. 

 It is obviously very distinct in form from the sacral vertebrae of the Megalosaurus. No 

 other reptile than one belonging to the order characterised by the peculiar structure of 

 the sacrum already described, could have yielded a detached vertebral centrum with 

 the remarkable modifications of the one under consideration. The modifications 

 detected in the entire sacrum of the Iguanodon in Mr. Saull's collection, justify the 

 reference of the vertebra above described to the sacrum of a young Iguanodon, and it 

 was probably the fourth of the series. 



Caudal Vertebra of the Iguanodon. Tab. VIII and IX. 



The typical vertebrae of this region — those, viz., with haemapophyses — are distin- 

 guished by the single haemapophysial surface at each end of the narrow inferior surface 

 of the centrum. The sides of the centrum are flat, or even slightly concave in the 

 vertical direction, though less so than in the antero-posterior direction. In a caudal 

 centrum, for example, in the Mantellian Collection, measuring 4 inches in length, and 

 5 inches 4 lines in depth at the middle of the side, if a pencil be laid vertically along 

 that part, an interval of between 1 and 2 lines separates its middle part from the bone. 

 Those equally great Wealden vertebrae which, on the contrary, have the middle of the 

 side of the body prominent, and the lower half only converging towards the under 

 surface, are from the tail of the Cetiosaurus. The posterior terminal articular surface 

 is rather more concave than in the dorsal vertebras ; but the difference is by no means 

 so marked as in the plano-concave vertebrae of the Cetiosaurus. The diapophyses* 

 Tab. VIII, pi d, of the anterior caudal vertebrae are comparatively short, but strong 

 and are continued from the base of the neurapophysis, or from, the contiguous part of 

 the centrum, or from both parts. 



The haemapophyses, or chevron bones, Tab. VIII, h, are not anchylosed to the centrum, 



* This process, in a certain proportion of the caudal series, is of the nature of a pleurapophysis, being 

 developed from a distinct centre, and articulated, in the young Iguanodon, as in the young Crocodile, by 

 suture to the rest of the vertebra. In succeeding vertebrae the homologous part is an exogenous process, 

 which gradually subsides to a ridge, where it is of the nature of a diapophysis ; and under such name, with 

 the above explanation, it seems to me most convenient to distinguish the transverse process of the caudal 

 vertebrae. 



