CRETACEOUS DINOSAURS. 271 



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 hues 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, arc soon worn 

 away, and can be seen only in a section of the apical part of the crown of an incom- 

 pletely 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 appear- 

 ance of interruption in the course of the calcigerous tubes, occasioned 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 inequality of texture and of density in 

 the dentine, which renders the broad and thick tooth of the Iguanodoji 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 perceptible undulating fibres, running 

 vertically to the surface of the tooth, form the only structure 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, 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. 



The microscopical examination of the structure of tho 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 enameled edge by the simple con- 

 trivance of arresting the calcifying process along certain tracts of the inner 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 following reflections were natural and just after a review of the external characters 



