16 FOSSIL REPTILIA OF THE 



but articulate with two contiguous vertebrae, crossing, and being somewhat wedged into, 

 the inferior interspace of those vertebrae ; in two of the caudal vertebras of the Maidstone 

 Iguanodon, there are two closely approximated haemapophysial surfaces, but in general 

 the haemal arch articulates with a single oblique triangular surface on each of the 

 contiguous extremities of the co-articulated vertebrae ; the haemapophyses being here 

 confluent at their vertebral as well as at their distal extremities. 



A caudal vertebra exhibiting this modification, in Mr. Holmes's collection, measures, 

 in the vertical diameter of the articular surface, 4 inches 9 lines ; in its transverse 

 diameter, 4 inches 6 lines ; the breadth of the inferior surface of the vertebra is 

 3 inches 3 lines. The interspace between the anterior and posterior haemapophysial 

 surface is 9 lines ; it is concave in the axis of the vertebra. The diameter of the spinal 

 canal is reduced in this vertebra to 9 lines. The transverse processes are of very 

 small size. The spinous process is broken off. We have seen that those of the sacral 

 vertebrae appear to have been short. There is reason to think that the spinous pro- 

 cesses increased in length for a certain distance as they receded from the sacrum, and 

 then diminished. Thus, in a caudal vertebra (No. ~^, Mantellian Collection, Brit. 

 Mus.), evidently anterior in position, by its size, by its oblique processes, and by the 

 place of development of its transverse processes from the base of the neural arch, the 

 spinous process is 5 inches in height, while in the six caudal vertebrae preserved in 

 natural sequence and relative position in the Mantellian Collection, the spines are more 

 than double that height, Tab. VIII. That the vertebra (No. 2130) is not a more 

 posterior caudal vertebra from a larger Iguanodon is shown by the relative thickness, 

 as well as position, of its transverse processes, as compared with the six caudal 

 vertebrae above mentioned, for their transverse processes sensibly diminish in every 

 diameter, and especially in vertical thickness, from the first to the sixth ; and, more- 

 over, it is evident that, in this short series, the spines decrease in height both forwards 

 from the third, as well as backwards, but more so in the latter direction. Thus the 

 spine, ns, of the first of these vertebrae is 14 inches high, of the third 15 inches, and of 

 the sixth 13 inches. These spines increase in breadth toward their summits, which 

 are truncated, and in contact with each other, partly from this expansion, partly from 

 the posterior ones being slightly bent forwards. One cannot witness this change of 

 character in so short a segment of the tail without a conviction that this appendage 

 must have been relatively shorter than in the Iguana. 



The first spine, besides being somewhat shorter, is more rounded off at its anterior 

 margin than the third, a difference which is still more obvious in the detached caudal 

 (No. 2130) above described ; but above its origin a thin trenchant plate is extended 

 for a short distance from the middle of the anterior margin : this character, which 

 calls to mind one that is present in a greater proportion of the vertebral column in the 

 Crocodilians, is more strongly developed in the second and third vertebrae. The 

 neurapophysial suture is more nearly obliterated in the sixth than in the first of this 



