324 BRITISH FOSSIL REPTILES. 



the latter : it is not more curved than the first. Still the difference, which is the 

 greatest I can detect in comparing the different ungual phalanges of the same CrocodUe, 

 is much less than that which is manifested by the depressed and compressed phalanges 

 hitherto deemed to characterise the hind and fore feet of the Iguanodon. I think it 

 more probable, therefore, that the second form of Wealden phalan.\ appertains to a 

 distinct species from the Iguanodon, and probably to a carnivoi'ous Saurian. 



The tbird form is that which, less depressed than the first and less curved than 

 the second, has been described as ihe horn of the Iguanodon. The outer border of the 

 lateral vascular grooves are vei'y slightly produced, and the grooves themselves 

 commonly sink into the substance of the Ijone, as they do in the great phalanges of 

 the Cetiosaurus. Some of these straight conical phalanges, e.g., those figured in 

 PI. 22, figs. 1, 2, 3, and 4, seem to be too large for the Ilylceosanrus. 



But I shall refrain, at present, from indulging in conjecture, however probable, 

 as to the species of reptile to which this third form of phalanx belongs, satisfied with 

 the present evidence of the nature of the bone itself, and that, if it ever formed part of 

 the skeleton of the Iguanodon, it belonged to the foot and not to the head : and 

 I shall conclude by briefly summing up the characters which ought to be borne in 

 mind when the idea of the little modern Iguana is associated, through similarity of 

 sound, with that of the great Iguanodon. 



Both articular ends of the vertebrae of the Iguanodon are nearly flat, thereby 

 difiering more from the concavo-convex vertebrte of the Iguana than those of any 

 existing Crocodile or Lizard do. 



The anterior ribs of the Iguanodon have a head, neck, and tubercle, and a double 

 articulation with the cervical and dorsal vertebras ; those of the Iguana and of every 

 other existing Lizard have no cervix or tubercle, and have only a single articulation 

 with the cervical and dorsal vertebrae. In this important modification of the anterior 

 ribs the Crocodile has a greater resemblance and closer afiinity to tlie Iguanodon than 

 the Iguana has. 



The height, breadth, and outward sculpturing of the neural arch of the dorsal 

 vertebrae of the Iguanodon, are characters wanting in the Iguana and all modern 

 Lizards, but are remotely approximated to in the dorsal vertebrae of the Crocodile, 

 which, however, are far fi'om presenting the expansion and complexity of the dorsal 

 neural arches in the Iguanodon. 



Five vertebras of unusual construction are anchylosed together to form the extended 

 sacrum of the Iguanodon : in the Iguana the small and simple sacrum consists of only 

 two slightly modified vertebrae ; in this respect it more closely resembles other Lizards, 

 and even the Crocodiles, than it does the Iguanodon. 



The important difference in the structure of the teeth of the Iguanodon and 

 Iguana, is pointed out in the Section on ' Cretaceous Reptiles,' pp. 269 — 271. 



