270 PALEONTOLOGY 



trie 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 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 opposite 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 follow- 

 ing reflections were natural and just, after a review of the 

 external characters of the dental organs of the Iguanodon, their 

 truth and beauty become still 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 peculiar functions, attended by compensation^ 

 adapted to shifting conditions of the instrument during different 

 stages of its consumption. And we must estimate the works 

 of nature by a different standard from that which we apply to 

 the productions of human art, if we can view such examples 

 of mechanical contrivance, united with so much economy of 

 expenditure, and with such anticipated adaptations to vary- 

 ing conditions in their application, without feeling a profound 

 conviction that all this adjustment has resulted from design 

 and high intelligence." 



All trace of dinosaurian reptiles disappears in the lower 

 cretaceous beds. 



