CLASS MAMMALIA 679 



largest portion of the tooth; and the cement (dotted) usually 

 covers the part of the tooth embedded in the tissues of the jaw. 

 The central pulp-cavity of the tooth contains nerves, blood- 

 vessels, and connective tissue. Teeth have an open pulp-cavity 

 during growth (Fig. 546, II), which in some cases continues 

 throughout life (Fig. 546, I). 



The teeth of fishes, reptiles, and amphibians are, with few 

 exceptions, all similar, and the dentition of these animals is 

 therefore said to be homodont. The dentition of mammals, on 

 the other hand, is almost always heterodont, there being usually 

 four kinds of teeth in each jaw: (i) the chisel-shaped incisors in 

 front (Fig. 518, 12), (2) the conical canines (c), (3) the anterior 

 grinding teeth or premolars (pm I pm 4), and (4) the posterior 

 grinding teeth or molars (m i). 



In most mammals the first set of teeth, known as the milk 

 dentition, is pushed out by the permanent teeth, which last 

 throughout the life of the animals. The milk molars are fol- 

 lowed by the premolars, but the permanent molars have no pred- 

 ecessors. 



It is customary to indicate the number of each kind of teeth 

 possessed by a mammal by a formula expressed in the form of a 

 fraction, of which the numerator refers to those in one half of 

 the upper jaw, and the denominator to those in one half of the 

 lower jaw. For example, the dog (Fig. 518) possesses three in- 

 cisors (i), one canine (c), four premolars (pm), and two molars 

 (m) in one half of the upper jaw, and three incisors, one canine, 

 four premolars, and three molars in one half of the lower 

 jaw. The dental formula of the dog is therefore written 



. 3 I 4 2 i r 3 I ' 4 2 ,-rn 



i ' 2 - c - ; pm - ; m- -, or in simpler form ^ The 



3i 43 3-I-4-3 



total number of teeth in the dog may be learned by adding these 



numbers and multiplying by two. 



The relation of the form of the teeth to the food habits of the 



animal may be shown by the following examples. The dolphins 



(Fig. 542) have a large number of sharp conical teeth adapted 



