122 REPORT— 1841. 



The teeth of the Iguaiiodon, 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 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, while 

 the crown is 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 description 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 sepa- 

 rated 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 deve- 

 loped 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 mag- 

 nifying power the form of transverse ridges, themselves notched, so as to re- 

 semble 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 farm. The tooth at first increases 

 both in breadth and thickness ; it then diminishes in breadth, but its thick- 

 ness goes on increasing ; in the larger and fully formed teeth, the fang de- 

 creases in every diameter, and sometimes tapers almost to a point. The 

 smooth unbroken surface of such fangs indicates that they did not adhere to 

 the inner side of the maxillae, as in the Iguana, but were placed in separate 

 alveoli, as in the Crocodile and Megalosaur : such support would appear, in- 

 deed, to be indispensable to teeth so 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 retained until nearly the whole of the 

 crown had yielded to the daily abrasion. In these teeth, however, the deep 

 excavation of the remaining fang plainly bespeaks the progress of the suc- 

 cessional tooth prepared to supply the place of the worn out grinder. At the 

 earlier stages of abrasion a sharp edge is maintained at the external part of 

 the tooth by means of the enamel which covers that surface of the crown ; 

 the prominent ridges upon that surface give a sinuous contour to the middle 

 of the cutting edge, whilst its sides are jagged by the lateral serrations: the 

 adaptation of this admirable dental instrument to the cropping and commi- 

 nution of such tough vegetable food as the Clathrurice and similar plants, 

 which are found buried with the Iguanodon, is pointed out by Dr. Buckland, 

 with his usual felicity of illustration, in his ' Bridgewater Treatise,' vol.i. p. 246. 



When the crown is worn away beyond the enamel, it presents a broad and 

 nearly horizontal grinding surface, and now another dental substance is brought 

 into use to give an inequality to that surface ; this is the ossified remnant of 

 the pulp, which, being firmer than the surrounding dentine, forms a slight 

 transverse ridge in the middle of the grinding surface : the tooth in this stage 

 has exchanged the functions of an incisor for that of a molar, and is prepared 

 to give the final compression, or comminution, to the coar"sely divided vege- 

 table matters. 



1 



