ON BRITISH FOSSIL. REPTILES. 123 



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 Iguans and other reptiles, and more easily Avorn aAvay : 

 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 in- 

 ternal and lateral sides of the pulp-cavity, and are confined to the dentine 

 forming the corresponding walls of the tooth : their diameter is f^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 ^^^^ffli of an inch, with inter- 

 spaces 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 afterwards pro- 

 ceed 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 direc- 

 tion 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 talce 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 calcigerous tubes, occa- 

 sioned by this modification of their secondary branches, is represented by the 

 irregularly dotted tracts in the figure. This modification must contribute, 

 with the medullary canals, though in a minor degree, in producing that ine-. 

 quality of texture and of density in the dentine, which renders the broad and 

 thick tootli of the Iguanodon more efficient as a triturating instrument. 



The enamel which invests the harder dentine, forming the outer side of the 

 tooth, presents the same peculiar dirty brown colour, when viewed by trans- 

 mitted light, as in most other teeth : very minute and scarcely perceptible un- 

 dulating fibres, running vertically to the surface of the tooth, is the only struc- 

 ture I have been able to detect 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 tlie plane of 

 tiie concentric lamellae, 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 whicii their more obvious characters had indicated them to have 

 been destined. 



To preserve a trenchant edge, a partial coating of enamel is applied ; and, 



