118 COMPARATIVE DENTAL. ANATOMY 



permanent and the tooth be of continuous growth. 



Three tissues enter into the composition of the 

 teeth of mammals. (1) The dentin, a dense os- 

 seous tissue like ivory, with radiating tubules 

 which is formed in the submucous tissue; (2) the 

 enamel, which covers the crown, which is devel- 

 oped from the epithelium of the mouth and con- 

 sists of calcined rods and intercemental sub- 

 stance; and (3) the cementum, which encases the 

 root, which is very similar to the true bone in 

 structure and organization. These three tissues 

 are found in most mammals in varying degrees 

 of completeness, except that enamel is absent 

 from the teeth of some low forms, as the ant- 

 eaters, sloths, and others. 



The relationship of the teeth of Mammals with 

 those of the Fishes and Reptiles below them is 

 well marked, and they present many resemblances 

 as well as some differences. The earliest as well 

 as some living lower mammals present reptilian 

 characteristics. Thus the teeth of the Dolphin 

 and other cetaceans are conical, curved, and more 

 or less devoid of enamel like reptilian teeth. 

 The highly specialized teeth of the higher mam- 

 mals differ as much from those of the lower mam- 

 mals as they do from the reptiles. There are 

 some important differences between the teeth of 

 reptiles and mammals. Thus reptiles never have 

 more than one root, while mammals may have two 



