MAMMALIA 



329 



roots. They range from simple one-cusped teeth like canines to those 



with a large number of cusps. 



The primitive types of cheek-teeth 



are provided with conical tuber- 

 cles, and are known as bunodont; a 



more highly specialized type of 



tooth has the tubercles connected 



by ridges, and is known as lopho- 



dont. There are usually two sets 



of teeth, a milk dentition and a 



permanent dentition, a condition 



known as diphyodont in contradis- . FIG. 169.-Teeth of dog.^ second 

 ,. . incisor; c, canine; pm 1, pm 4, nrst and 



tmction to the condition char- fourth pre mol a rs; m 1, first molar. 



acteristic of the lower vertebrates, (From Hegner, after Shipley and Mac- 



which have but one set of teeth Bride -) 



(monophyodoni) . In the lowest mam- 

 mals there is no second dentition, or 

 only a partial replacement of the first 

 by the second set. Many mammals 

 also have degenerate dentition, involv- 

 ing an entire loss of teeth or merely a 

 loss of incisors, or canines, or some of 

 the molars. 



A typical tooth (Fig. 170) consists of 

 three kinds of tissue: enamel, dentine, 

 and cement. The enamel is derived 

 from the epithelium of the mouth cav- 

 ity and is therefore ectodermal; the 

 other constituents are dermal in origin. 

 The teeth arise as tooth-germs quite 

 independent of the jaws and later be- 

 come imbedded in sockets of the 

 latter. The dental epithelium is at 



FIG. 170. Diagrammatic section of various forms of teeth. I, incisor or tusk 

 of elephant with pulp cavity open at base. II, human incisor, during develop- 

 ment, with pulp cavity open at base. Ill, completely formed human incisor, 

 opening of pulp cavity small. IV, human molar with broad crown and two roots. 

 V, molar of ox, enamel deeply folded and depressions filled with cement. Enamel, 

 black; pulp, white; dentine, horizontal lines; cement, dots. (From Hegner, after 

 Flower and Lydekker.) 



