Class 6. Mammalia. 



483 



it may be low and broad^ provided witli rounded or pointed tubercles, 

 or it may be much compressed and serrate at the edge ; or again, it 

 may be traversed by marked transverse or longitudinal ridges 

 separated by valleys, which may be so deep as to reach the base 

 of the crown {e.g., in the molar teeth of the Elephant). Perpendicular 

 furrows may also occur on the sides of the teeth ; usually the deeper 

 folds are partially or completely filled with cement (in the Horse and 

 Elephant). During use, the enamel, especially in plicate teeth, is very 

 frequently worn away at all the projecting points, and the subjacent 

 dentine is thus laid bare ; at the grinding surface, therefore, may be 

 seen islands of dentine surrounded by slightly elevated enamel 

 borders, and the latter are often again surrounded by cement 

 (especially in teeth of herbivorous animals) . The crowns and roots of 

 many teeth are of almost equal length ; sometimes one, sometimes the 

 other may, however, be the longer. The former especially the case with 

 much folded teeth, which are subjected to considerable wear and tear ; 

 in these the root (or roots) is often very short ; the crown, on the 

 contrary, is very long, but it projects from the jaw for only part of its 

 length, and gradually, as the free end is worn away, the tooth is pushed 

 out {e.g., in the molars of the Horse). The crown has often begun to 

 wear down before the root is formed ; in other cases tbere is 

 actually no development of a root; as the crown is worn 

 away above, growth takes place below, and never ceases ; such teeth 

 are said to grow from persistent pulps (the incisors of E.odents, the 

 molars of many of the same group, the canines of the Boar, etc.). 



The teeth are arranged in a single row along the edge of 

 premaxilla, maxilla, and mandible ; those of 

 designated incisors, the anterior 

 teeth of the maxilla, next to these, 

 canines; the rest, molars; 

 in the lower jaw the teeth which 

 bite just in front of the upper 

 canines are known by that name 

 also ; those anterior, the incisors ; 

 those behind, the molars. In most 

 placental* Mammals the number of 

 teeth on each side of the jaw,t in 

 the second or permanent deii- 

 t i t i o n, is not more than eleven, 

 three incisors (i^, i^, v"), one canine 

 (c), seven molars, of which the four 

 anterior are termed premolars 



the premaxilla are 



Ca 



Fig. 393. Dentition of a Mole {com- 

 plete dental formnla) ; the milk teeth are 

 drawn in outline above or below the cor- 

 responding permanent ones, i^ third 

 incisor, c canine, p premolar, m molar. — 

 After Ch. Tomes. 



* The placental Mammals include all Mammalia excepting Monotremes and Mar- 

 supials (see p. 493). 



t One premaxilla and one maxilla are regarded as half of the upper jaw. 



I I 2 



