52 Royal Society. 



is the peculiar construction of the anterior part of the lower jaw, 

 which forms the sj^^mphysis. This process, instead of being con- 

 tinued round the front of the mouth and beset with teeth, us in all 

 other saurians, is edentulous, and extends into a procumbent scoop- 

 like expansion, very analogous to the symphysial portion of the 

 lower jaw in the Sloths, and especially to that of the colossal extinct 

 Edentata — the Mylodons. Along the external surface of this dentary 

 bone there is a row of very large vascular foramina ; and the sym- 

 physis also is perforated with numerous openings for the passage of 

 blood-vessels and nerves sent off from the great dental canal. The 

 unusual number and magnitude of these foramina indicate a great 

 development of the integuments and soft parts with which the bone 

 was invested, and also the large size of the under lip. 



The upper jaw, of which a considerable portion discovered by the 

 author is in the British Museum, confirms the inferences deduced 

 from the teeth and dentary bone of the lower maxilla. 



The author, with the able assistance of Dr. A. G. Melville, instituted 

 a comparison between all the teeth of the Iguanodon to which he could 

 obtain access, and those of recent saurians ; and the result of the 

 investigation is detailed. The new light shed on the structure and 

 functions of the dental organs, confirms, in every essential particular, 

 the inferences deduced by the author from the detached teeth alone, 

 in his memoir of 1825 ; and it also reveals an extraordinary devia- 

 tion from all known types of reptilian organization, and which could 

 not have been predicated ; namely, that this colossal reptile, which 

 equalled in bulk the gigantic Edentata of South America, and like 

 them was destined to obtain support from comminuted vegetable sub- 

 stances, was also furnished with a large prehensile tongue and fleshy 

 lips, to serve as instruments for seizing and cropping the foliage and 

 branches of trees ; while the arrangement of the teeth as in the ru- 

 minants, and their internal structure, which resembles that of the 

 molars of the Sloth tribe in the vascularity of the dentine, indicate 

 adaptations for the same purpose. 



Among the physiological phsenomena revealed by Palaeonto- 

 logy, there is not a more remarkable one than this modification 

 of the type of organization peculiar to the class of reptiles, to meet 

 the conditions required by the economy of a lizard placed under 

 similar physical relations, and destined to effect the same general 

 purpose in the scheme of nature, as the colossal Edentata of former 

 ages, and the large herbivorous mammalia of our own times. 



From the facts detailed, the author is led to consider the specimen 

 described in his memoir of 1841, as being probably the lower jaw of a 

 young Iguanodon (but the true nature of which, from the absence of 

 the crowns of the teeth, was doubtful) belonging to the same family, 

 but referable to a distinct genus or subgenus ; and he proposes the 

 name oi Regnosaurus Northamptoni for that remarkable fossil saurian. 

 The communication was illustrated by several drawings of the speci- 

 mens described. 



