IGUANODON. 251 



introduction of a modification into the texture of the dentine, by 

 which it is rendered softer than in the existing Iguanas 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 calcige- 

 rous 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 i^^oth 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 griooth 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 afterwards 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 remark- 

 able 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 appearance of interruption in the 

 course of the calcigerous tubes occasioned by this modification of 



