252 IGUANODON. 



their secondary branches is represented by the irregularly dotted 

 tracts in the figure (PI. 71). 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 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 transmitted light as in most other teeth : very minute and 

 scarcely perceptible undulating fibres, running vertically to the sur- 

 face of the tooth, is the only structure I have been able to detect 

 in it. 



The remains of the pulp in the contracted cavity of the com- 

 pletely formed tooth are converted into a dense but true osseous sub- 

 stance, characterized by minute elliptical radiated cells, whose long 

 axis is parallel with the plane of the concentric lamellse, which sur- 

 round 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 for 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 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 pre- 

 pared 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 charac- 

 ters of the dental organs of the Iguanodon, their truth and beauty 

 become 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 



