GENERAL STRUCTURE. 



1247 



cases, however, as in the anterior cutting-teeth of the Rodents and 

 the tusks of the Elephant, the root remains permanently open be- 

 low, and the tooth consequently continues to grow throughout the 

 life of the animal. In such circumstances the teeth are said to have 

 persistent pulps. The anterior teeth (fig. 1126, c, i), with some 

 rare exceptions, are of simple structure and have but a single root ; 

 but the hinder ones {ibid., pm, m) very generally have more or less 

 complex crowns, which may be supported by from two to four roots ; 

 such division of the roots being unknown outside the Mammalian 

 class. In many forms the summits or sides of the crowns of the 

 hinder teeth may be interpenetrated by deep re-entering folds of 

 enamel, which may be filled up with cement ; these folds being espe- 



Fig. 1126.— Teeth of the right side of the lower jaw of the Chimpanzee, z, Incisors ; 

 c, Canine ; pm, Premolars ; ;«, True molars. (After Owen.) 



daily developed in many Rodents and Ungulates. From this 

 structure it will naturally result that when a horizontal section of the 

 crown of such a tooth is made by the wearing of the upper against 

 the lower series, an extremely complex pattern will appear, as will 

 be seen in the figures of the cheek-teeth of the above-mentioned 

 groups which are given below. Much more rarely, as in the Horse, 

 there may be an infolding of the enamel in the summits of the 

 crowns of the anterior teeth. 



With the exception of the above-mentioned edentulous forms, in 

 all existing Mammals one definite set of teeth, which is almost 

 always constant in number, is developed, and this set when it ap- 

 pears usually persists throughout the remaining portion of the life 

 of its owner. In a large number of species this is the only set ever 

 developed ; and such species, or groups of species, are consequently 

 said to be Monophyodont In the greater majority of Mammals, 

 however, the development of occasionally only one, but usually of 

 the greater number of the anterior teeth of this permanent set is 



