TEETH. 



893 



creases in every diameter, and sometimes 

 tapers almost to a point. The smooth un- 

 broken surface of such fangs indicates that 

 they did not adhere to the inner side of 

 the maxilla?, as in the Iguana, but were 

 placed in separate alveoli, as in the Cro- 

 codile and Megalosaur ; such support would 

 appear, indeed, to be indispensable to teeth 

 so worn by mastication as those of the Igu- 

 anodon. 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 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 fig. 57 1 ., plainly bespeaks the pro- 

 gress 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 

 ClathraricB 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 Iguana? 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 ia 

 -^th 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 

 ^, 00 th 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 Jguanodon 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 transmitted light, as in most 

 other teeth : very minute and scarcely per- 

 ceptible undulating fibres, running vertically 

 to the surface of the tooth, is the only struc- 

 ture I have been able to detect in it. 



The cement is simply and minutely cellular 

 upon the crown of the tooth, but it exhibits 

 the radiated cells at the base of the tooth. 



The remains of the pulp in the contracted 

 cavity of the completely formed tooth are 

 converted into a dense but true osseous 

 substance, characterised by minute elliptical 

 radiated cells, whose long axis is parallel with 

 the plane of the concentric lamellae, which 

 surround the few and contracted medullary 

 canals in this substance. 



