270 ANATOMY OF VERTEBRATES. 



of the tooth with the jaw constitute a normal mode of attach- 

 ment. Each tooth has its particular socket, to which it firmly 

 adheres by the close co-adaptation of their opposed surfaces, and 

 by the firm adhesion of the alveolar periosteum to the organised 

 cement which invests the fang or fangs of the tooth. 



True teeth implanted in sockets are confined to the maxillary, 

 premaxillary, and mandibular or lower maxillary bones, and form 

 a single row in each. They may project only from the premax- 

 illary, as in the Narwhal, or only from the lower maxillary as in 

 the Ziphius ; or be apparent only in the lower jaw, as in the 

 Cachalot ; or be limited to the superior and inferior maxillaries, 

 and not present in the premaxillaries, as in the true Ruminants 

 and most Bruta. 



Mammalian teeth usually consist of hard un vascular dentine, 

 fig. 210, d, defended at the crown by an investment of enamel, 

 ib. e, and everywhere surrounded by a coat of cement, ib. c. 

 The coronal cement is of extreme tenuity in Man, Quadrumana, 

 and terrestrial Carnivora; it is thicker in the Herbivora, espe- 

 cially in the complex grinders of the Elephant, fig. 289, and is 

 thickest in the teeth of the Sloth, Megatherium, Dugong, Walrus, 

 and Cachalot. Vertical folds of enamel and cement penetrate the 

 crown of the tooth in most Rodents and Ungulates, characterising 

 by their various forms the genera ; but these folds never converge 

 from equidistant points of the circumference of the crown towards, 

 its centre. The teeth of Bruta have no true enamel; this is 

 absent likewise in the molars of the Dugong and of the fully de- 

 veloped teeth of the Cachalot. The tusks of the Narwhal, Walrus, 

 Dinothere, Mastodon, and Elephant, consist of modified dentine, 

 which, in the last two great proboscidian animals, is properly 

 called ' ivory,' and is covered by cement. 



The Dolphins and Armadillos present little variety in the shape 

 of the teeth in the same animal, and this sameness of form is 

 characteristic of Monophyodonts ; subject, like the successional 

 character, to such exceptions as are exemplified in Choloepus 

 didactylus, fig. 215, and in Dasypus 9-cinctus, the milk-teeth of 

 which are figured in cxxxn", p. 254. 



In most other Mammals particular teeth have special forms for 

 special uses : thus the front teeth, from being commonly adapted 

 to effect the first coarse division of the food, have been called 

 cutters or incisors ; and the back teeth, which complete its com- 

 minution, grinders or molars : large conical teeth, situated behind 

 the incisors, and adapted by being nearer the insertion of the 

 biting muscles, to act with greater force, are called holders, 

 tearers, laniaries, or more commonly canine teeth, from being well 



