132 REPORT — 1841. 



physes being here confluent at their vertebral as m ell as at their distal ex- 

 tremities. 



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

 measures, in the vertical diameter of the articular surface, •i inches 9 lines ; 

 in its transverse diameter, 4 inches 6 lines ; the breadth of the inferior sur- 

 face 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 pro- 

 cess 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 processes increased 

 in length for a certain distance as they receded from the sacrum, and then dimi- 

 nished. Thus, in a caudal vertebra (No. 2^, Mautellian Collection), 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 Collec- 

 tion, the spines are more than double that height. 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 pro- 

 cesses, as compared with the six caudal vertebras above mentioned, for their 

 transverse processes sensibly diminish in every diameter, and especially in 

 vertical thickness, from the first to the sixth ; and, moreover, 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 of the first of these vertebrae is M 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 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 vertebree. The neurapophysial suture is more nearly ob- 

 literated in the sixth than in the first of this instructive series, or in the more 

 anterior and detached caudal vertebra. The following are dimensions of the 

 detached anterior caudal (No. 1), and of the first (No. 2) and last (No. 3) 

 of the series of six : — 



Antero-posterior diameter of centrum , . . 

 Vertical diameter of articular surface . . . 

 Transverse diameter of articular surface . . 

 From under part of centrum to upper end 1 r £• 



of posterior articular process . . . . j 5 8 4 



From upper end of posterior oblique pro- 1 



cess to the summit of spine . . . . J '^ " 14 10 6 

 Antero-posterior diameter of base of spine .13* 17 14 

 Antero-posterior diameter of summit of spine 2 2 2 2 6 

 * The anterior basal ridge of this vertebra is broken away. 



