116 FOSSIL REPTILIA OF THE 



increasing ; in the larger and fully formed teeth, the fang decreases 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, indeed, 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 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 wdiich 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 comminution 

 of such tough vegetable food as the Clathrarice 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 vegetable matters. 



The marginal edge of the incisive condition of the tooth, and the median ridge of 

 the molar stage, are more effectually established by the introduction of a modification 

 into the texture of the dentine, by which it is rendered softer than in the existing 

 Iguanse and other reptiles, and more easily worn away : this is effected by an arrest of 

 the calcifying process along certain cylindrical tracts of the pulp, which is thus con- 

 tinued, in the form of medullary canals, analogous to those in the soft dentine of the 

 Megatherium's grinder, from the central cavity, at pretty regular intervals, parallel 

 with the calcigerous tubes, nearly to the surface of the tooth. The medullary canals 

 radiate from the internal and lateral sides of the pulp cavity, and are confined to the 

 dentine forming the corresponding walls of the tooth : their diameter is y^y-o-th of an 

 inch : they are separated by pretty regular intervals equal to from six to eight of their 

 own diameters ; they sometimes divide once in their course. Each medullary canal is 

 surrounded by a clear space ; its cavity was occupied in the section described by a 

 substance of a deeper yellow colour than the rest of the dentine. 



The calcigerous tubes present a diameter of yyVo-oth of an inch, with interspaces 



