IGUANODON. 249 



The teeth of the Iguanodon, though resembling most closely 

 those of the Iguana, do not present an exact magnified image of 

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

 more compHcated 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, contracted 

 and subcylindrical ; the crown expanded, and smoothly convex on the 

 inner side ; when first formed (PL 70, fig. 5) it is acuminated, com- 

 pressed, its sloping sides serrated, and its external surface traversed 

 by a median longitudinal ridge, and coated by a layer of enamel, but 

 beyond this point the description of the tooth of the Iguanodon indi- 

 cates characters peculiar to that genus. In most of the teeth that 

 have hitherto been found, three longitudinal ridges traverse the outer 

 surface of the crown, one on each side of the median primitive ridge ; 

 these are separated from each other and from the serrated margins of 

 the crown by four wide and smooth longitudinal grooves. The rela- 

 tive width of these grooves varies in different teeth ; sometimes a 

 fourth small longitudinal ridge is developed on the outer side of the 

 crown as in the small anterior Iguanodon's tooth in Plate 62 a, 

 fig. 5, a. 



The marginal serrations, which, at first sight, appear to be 

 simple notches, as in the Iguana, present under a low magnifying 

 power the form of transverse ridges, themselves notched, as shown in 

 PI. 62 A., fig. 5 e.(l) so as to resemble the mammillated margins of 

 the unworn plates of the elephant's grinder : slight grooves lead from 

 the interspaces of these notches upon the sides of the marginal ridges. 

 These ridges or dentations do not extend beyond the expanded part of 

 the crown : the longitudinal ridges are continued farther down, espe- 

 cially the median ones which do not subside till the fang of the tooth 

 begins to assume its subcylindrical form. 



The tooth at first increases both in breadth and thickness ; it then 

 diminishes in breadth, but its thickness goes on increasing (as shown 



(1) This figure, and those numbered 5, 6, 8, 9 and 10, PI. 62 a., are zincographed from 

 drawings by Mr. Dinkel kindly communicated to me by Dr. Manteli for the present work. 



