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 ridged 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 comminution of such tough vegetable food as the Clathrarial 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 pre sents a broad and nearly horizontal grinding surface (fig. 79), 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 A worn tooth of exchanged the functions of an incisor for that the Iguanodon. of a molar, and is prepared to give the final compression, or comminution, to the coarsely divided vege table 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 Iguanw 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 dentinal tubes, nearly to the surface of the tooth. The medullary canals radiate from the internal (upper jaw) or external (lower jaw) sides of the pulp-cavity, and are confined to the dentine forming the corresponding walls of the tooth. Their diameter is 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 dentinal 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, hut afterwards proceed in slight and regular primary curves, or in nearly straight lines to the periphery of the tooth. The secondary undulations of each tooth are regular, and very minute. The branches, both primary and secondary, of the dentinal tubes are sent off from the concave side of the main inflections ; the minute secondary branches are remark able at certain parts of the tooth for their flexuous ramifica tions, 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 appearance of
interruption in the course of the dentinal tubes, occasioned by this modification of their secondary branches, is represented by the irregularly dotted tracts in the figure. This modifica tion must contribute, with the medullary 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.
The enamel which invests the harder dentine, forming the ridged side of the tooth, presents the same peculiar dirty brown colour, when viewed by transmitted light, as in most other teeth. Very minute and scarcely perceptible undulating fibres, running vertically to the surface of the tooth, form the only discernible structure in it.
The remains of the pulp in the contracted cavity of the completely formed tooth are converted into a dense but true osseous substance, characterized by minute elliptical radiated cells, whose long axis is parallel with the plane of the concen tric lamella, which surround the few and contracted medullary canals in this substance.
The microscopical examination of the structure of the Iguanodon's teeth thus contributes additional evidence of the perfection of their adaptation to the offices to which their more obvious characters had indicated them to have been destined.
To preserve a trenchant edge, a partial coating of enamel is applied ; and, that the thick body of the tooth might be worn away in a more regularly oblique plane, the dentine is rendered softer as it recedes from the enamelled edge, by the simple contrivance of arresting the calcifying process along certain tracts of the opposite wall of the tooth. When attrition has at length exhausted the enamel, and the tooth is limited to its function as a grinder, a third substance has been prepared in the ossified remnant of the pulp to add to the efficiency of the dental instrument in its final capacity. And if the follow ing reflections were natural and just, after a review of the external characters of the dental organs of the Iguanodon, their truth and beauty become still more manifest as our knowledge of their subject becomes more particular and exact : " In this curious piece of animal mechanism we find a varied adjustment of all parts and proportions of the tooth, to the exercise of peculiar functions, attended by compensations adapted to shifting conditions of the instrument during different stages of its consumption. And we must estimate the works of nature by a different standard from that which we apply to the productions of human art, if we can view such examples of mechanical contrivance, united with so much economy of expenditure, and with such anticipated adaptations to vary ing conditions in their application, without feeling a profound conviction that all this adjustment has resulted from design and high intelligence." All trace of dinosaurian reptiles disappears in the lower cretaceous beds.