250 IGUANODON. 



in fig. 5, c) ; in the larger and fully formed teeth, the fang decreases in 

 every diameter, and sometimes, as in the tooth fig. 5 d, tapers 

 almost to a point. A fracture of this tooth shows that the pulp was 

 not entirely solidified, but that its cavity had continued open at the 

 thickest part of the tooth. 



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

 appear, by many specimens like that in PL 70, fig. 3, 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, represented in profile in the figure (ib. fig. 3) plainly 

 bespeaks the progress of the successional 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 (PL 70, figs. 1 & 2) : the adaptation 

 of this admirable dental instrument to the cropping and comminution 

 of such tough vegetable food as the Clathrarise 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 coarsely divided vegeta- 

 ble matters (1). 



The marginal edge of the incisive condition of the tooth and the 

 median ridge of the molar stage are more effectually established by the 



(1) A tooth, presented to me by Mr. Dixon of Worthing, exhibits this modification of its 

 grinding surface. 



