DINOSAURIA. 295 



but differ in the greater relative thickness of the crown, its 

 more complicated external surface, and, still more essentially, 

 in a modification of the internal structure, by which the 

 Iguanodon equally deviates from every other known reptile. 



As in the Iguana, the base of the tooth is elongated and 

 contracted ; the crown expanded and marginally notched ; 

 when first formed it is acuminated, compressed, its sloping- 

 sides serrated, and one surface, external in the upper jaw, 

 internal in the lower jaw, is traversed by a median longi- 

 tudinal ridge, and coated by a layer of enamel ; but beyond 

 this point the description of the tooth of the Iguanodon 

 indicates characters peculiar to that genus. The median 

 ridge is most produced in the teeth of the upper jaw, and 

 on each side of it, in both upper and lower teeth, are 

 one or two lower ridges ; these are separated from each 

 other and from the serrated margins of the crown by wide 

 and smooth longitudinal grooves. The marginal serrations 

 which, at first sight, appear to be simple notches, 

 as in the Iguana, present under a low magnify- 

 ing power (fig. 101), the form of transverse 

 ridges, themselves notched, so as to resemble 

 the mammilated margins of the unworn plates 

 of the elephant's grinder. The base of the 

 crown, soon contracts to a round, bent, smooth Marginal ridges 

 fang. These did not merely adhere to the inner fthelguano- 

 side of the alveolar parapet, as in the Iguana, don ' ma s n - 

 but were placed in separate alveoli ; such support being- 

 indispensable to teeth so used and worn by mastication as 

 those of the Iguanodon. 



The apex of the tooth soon begins to be worn away, and 

 it would appear, by many specimens, that the teeth were re- 

 tained until nearly the whole of the crown had yielded to 

 the daily abrasion. In these teeth, however, the deep excava- 

 tion of the remaining fang plainly bespeaks the progress of 



