The apex of the tooth soon begins to be worn away, and it would appear, by many specimens that have been found, 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 dig. 571., plainly bespeaks the pro. greys 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. The adaptation of this admirable dental instrument to the cropping and com minution 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 il lustration, in his " Bridgewater Treatise," vol. i. p. 246.
When the crown is worn away beyond the enamel, it presents a broad and nearly hori zontal 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 Iguanm 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 continued, 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 corre sponding walls of the tooth; their diameter is of an inch ; they are separated by pretty regular intervals, equal to from six to eight of their own diameters ; they some times divide once in their course. Each
medullary canal is surrounded by a clear sub stance ; 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 of an inch, with interspaces equal to about four of their diameters. At the first part of their course, near the pulp-cavity, they are bent in strong undulations, but after wards proceed in slight and regular primary curves, or in nearly straight lines, to the periphery of the tooth. When viewed in a longitudinal section of the tooth, the concavity of the primary curvature is turned towards the base of the tooth ; the lowest tubes are inclined towards the root, the rest have a general direction at right angles to the axis of the tooth ; the few calcigerous tubes, which proceed vertically to the apex, are soon worn away, and can be seen only in a section of the apical part of the crown of an incompletely developed tooth. The secondary undulations of each tooth are regular and very minute. The branches, both primary and secondary, of the calcigerous tubes are sent off from the concave side of the main inflections ; the minute secondary branches are remarkable at certain parts of the tooth for their flexuous ramifications, anastomoses, and dilatations into minute calcigerous cells, which take place along nearly parallel lines for a limited extent of the course of the main tubes. The ap pearance of interruption in the course of the calcigerous tubes, occasioned by this modi fication of their secondary branches, is repre sented by the irregularly dotted tracts in the figure of the dental structure of this ancient reptile given in my " Odontography." This modification must contribute, with the me dullary canals, though in a minor degree, in producing that inequality of texture and of density in the dentine, which renders the broad and thick tooth of the Iguanodon more efficient as a triturating instrument.