560 ZOOLOGY SECT. 



open below (7), and the tooth constantly growing from the base as 

 it becomes worn away at the crown ; such teeth are said to have 

 persistent pulps. 



Usually Mammals have two distinct sets of teeth developed, 

 the milk and permanent dentitions, but sometimes there is only 

 one, and accordingly we distinguish diphyodont and monopJiyodont 

 Mammals : in nearly all of the latter, however, another set are 

 developed, though they early become absorbed or remain in the 

 condition of functionless vestiges ; and in a considerable number 

 of groups it has been found that more than two sets of teeth 



dc 

 Ac 



elm .3 



FIG. 1184. Milk- and permanent dentition of upper (I) and lower (II) jaw of the Dog (Cants 

 familiaris), with the symbols by which the different teeth are commonly designated. (After 

 Flower and Lydekker.) 



are formed, only one, or at most (in diphyodont forms) two, of 

 these sets becoming fully developed. The milk-teeth in Mam- 

 mals with typical diphyodont dentition sometimes disappear at 

 an early stage, and sometimes do not become replaced by the 

 permanent teeth till long after birth. Some Mammals have the 

 teeth almost indefinite in number, e.g., the Dolphins and Porpoises, 

 in which they are all uniform (homodont) and not divided into 

 sets (Fig. 1185). In the typical dentition there are forty-four 

 teeth, viz., three incisors on each side, one canine and seven pre- 

 molars and molars above and below. The incisors (Fig. 1184, i.) of 

 the upper jaw are to be distinguished as being the teeth that 

 are lodged in the premaxilJa3 ; the incisors of the lower jaw are 



