CRETACEOUS FORMATIONS. 115 



teeth, resembling in shape those of the Iguana, but in structure differing from the 

 teeth of that and every other known reptile, and unequivocally indicating the former 

 existence in the Dinosaurian Order of a gigantic representative of the small group of 

 living lizards which subsist on vegetable substances. 



The important difference which the fossil teeth presented in the form of their 

 grinding surface was pointed out by Cuvier,* of whose description Dr. Mantell adopted 

 a condensed view in his ' Illustrations of the Geology of Sussex,' 4to, 1827, p. 72. The 

 combination of this dental distinction with the vertebral and costal characters, which 

 prove the Iguanodon not to have belonged to the same group of Saurians as that which 

 includes the Iguana and other modern lizards, rendered it highly desirable to ascertain 

 by the improved modes of investigating dental structure, the actual amount of corres- 

 pondence between the Iguanodon and Iguana in this respect. This I have done in my 

 general description of teeth of reptiles,! from which the following description is 

 abridged : — 



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 complicated external surface, and, still more essen- 

 tially, 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 smoothly convex on the inner side ; when first formed it is acuminated, 

 compressed, 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 descrip- 

 tion of the tooth of the Iguanodon indicates 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 relative width of these grooves varies in 

 different teeth ; sometimes a fourth small longitudinal ridge is developed on the outer 

 side of the crown. 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 trans- 

 verse ridges, themselves notched, 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 further down, especially 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 



* Ossemens Fossiles, 1824, vol. v, part ii, p. 351. 



f Odontography, part ii, p. 249 ; and Transactions of the British Association, 1838. 



