286 MAMMALIA. 



are confined to the maxillae, prsemaxillse, and mandible. The external 

 part of the tooth (i.e., the part which projects from the gum, and is 

 called the crown, as opposed to the root) is covered with the harder 

 enamel, which consists of prisms arranged at right angles to the 

 cavity of the tooth (pulp cavity). Two kinds of teeth may be 

 distinguished (1) Simple teeth (d. simplices), in which the layer of 

 enamel forms a simple cap; (2) complicated teeth (d. complicati), in 

 which the enamel is folded and penetrates into the dentine. "When 

 simple or complicated teeth are connected together by osseous tissue 

 (cement), they are called composite teeth (d. compositi Hare, Ele- 

 phant). Rarely, and only in those cases in which the dentition is 

 used, as in the Crocodile, as a prehensile or cutting apparatus, are 

 the teeth in all parts of the jaws alike, having the form and function 



FIG. 673. Skull of Balaena mysticetus with the whalebone (regne animal). 



of prehensile conical teeth (Dolphin). As a rule, they are distin- 

 guished according to their position in the anterior, lateral, and 

 posterior parts of the jaw as incisors, canines, and grinders (back 

 teeth). The incisors are chisel-shaped, and serve to cut the food; 

 in the upper jaw they belong exclusively to the prsemaxillary bones. 

 The canines, which are placed to the sides of the incisors, one in each 

 half of the jaw, are conical or hooked, and serve principally 

 as weapons for attack and defence; not unfrequently, however 

 they are absent (Ruminants, Rodents), and there is a wide gap 

 (diastema) between the incisors and the grinders. The latter are 

 specially adapted for the finer mastication of the food, and their 

 crowns are usually provided with a tuberculated or grinding surface. 

 The teeth either last throughout life, and the dentition is not renewed 

 (Monophyodonta : Edentates, Cetaceans), or there is a single change 

 of teeth (Diphyodonta) (fig. 674). In the latter case the teeth which 



