234 



CHEMICAL COMPOSITION OF TEETH. 



reptiles, completely fills those canals, which are compa- 

 ratively wide, whilst their parietes are of extreme tenuity. 



The examples are extremely few, and, as far as I know, 

 are peculiar to the class Pisces, of calcified teeth, which 

 consist of a single tissue, and this is always a modifica- 

 tion of dentine. The large pharyngeal teeth of the 

 wrasse {Labrus) consist of a very hard kind of dentine. 



The next stage of complexity is where a portion of the 

 dentine is modified by vascular canals. Teeth, thus 

 composed of dentine and vasodentine, are very common 

 in fishes. 



The hard dentine is always external, and holds the 

 place and performs the office of enamel in the teeth of 

 higher animals. The grinding teeth of the dugong, and 

 the conical teeth of the great sperm whale, are examples 

 of teeth composed of dentine and cement, the latter tissue 

 forming a thick external layer. 



In the teeth of the sloth, and its great extinct conge- 



* Selected from the analytic tables given in the author's "Odoutogra- 

 phy," 4to. vol. i. pp. Ixii., Ixiv. (1840.) 



