INTRODUCTION. Ixxiii 



So likewise the office of teeth is performed, in the Articulate Classes, 

 by parts (modified as to form) of the criistaceous and chitinous 

 integuments. But I know of no transitional or intermediate structures, 

 such as Heusinger alludes to, between teeth and nails, horns or hair. 

 The lamellar disposition traceable in the texture of the hardest 

 dentine, is much more closely similar to that of bone, especially 

 to the concentric plates surrounding the Haversian Canals, than to 

 the texture of nails. The structure of the tooth of the Oryc- 

 teropus, is essentially like that of all true teeth : the apparent 

 resemblance which it presents to the horn of the Rhinoceros, or 

 to Baleen, arises from its being compounded of many minute 

 parallel and elongated denticles. 



And the close resemblance in intimate structure and chemical 

 composition between true teeth and bones being established, it 

 may be observed that the osseous tissue is not confined to the 

 endo-skeleton : it is developed largely to form the exo-skeleton 

 in many Fishes, in the Loricate Reptiles, and even in the Mamma- 

 lian Class, as, for example, in the Armadillos (Dasypus), where, 

 to strengthen the integument, bone is substituted for horn, which 

 forms the scaled armour of the allied Pangolins (Manis,) Now the re- 

 lation of the tooth of the Armadillo to that of the Ornithorhynchus is 

 precisely analogous to that which subsists between the osseous 

 plates of the Armadillo and the corneous scales of the Pangolin ; 

 but this relation no more establishes identity of tissue or 

 system of tissues in the one case than in the other. 



The general form of the dental matrix and its relation with its 

 calcified product, bear a close analogy with those of the formative organ 

 of hairs, bristles, and other productions of the epidermal system. 

 In these the papilla, or pulp, is developed from the external 

 skin ; in the teeth from the mucous membrane, or internal skin. 



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