xm PHYLUM CHORDATA 543 



In the teeth of most Mammals distinct roots are formed, each with a 

 minute opening leading into the pulp-cavity (Fig. 1193, /// V) ; but 

 in some there are no roots, the pulp-cavity being widely open below 

 (/), and the tooth constantly growing from the base as it becomes worn 

 away at the crown ; such teeth are said to have persistent pulps. 



UsuaUy 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 monophyodont 

 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 



<a?e 



f / 2.3 



FIG. 1196. Milk- and permanent dentition of upper (/) and lower (//) jaws of the Dog (Canis 

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

 (After Flower and Lydekker.) 



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

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

 sets becoming fully developed. The milk-teeth in Mammals 

 with typical diphyodont dentition sometimes disappear at an 

 early stage, and sometimes do not become replaced by the per- 

 manent 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. 1197). 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. 1196, i.) of 



L L 2 



