THE TEETH OF VERTEBRATES 83 



The first set remains permanently, is more or less 

 continually growing, and is never displaced by a 

 second set. The diphyodont mammals have first 

 a deciduous set which supplies the dental wants 

 of the individual during the first years of exist- 

 ence. It is then shed and replaced by the per- 

 manent set, which should remain during the life- 

 time of the individual. The teeth which succeed 

 the deciduous teeth are called succedaneous teeth, 

 but additional teeth are sometimes produced 

 which have no deciduous predecessors. Thus in 

 man the ten anterior teeth are shed and replaced 

 by permanent teeth, but there are also six addi- 

 tional teeth, the true molars, in each jaw which 

 erupt without displacing any deciduous teeth. 



The monophyodonts are usually homodonts, — 

 i.e., the teeth are all alike and of the same form 

 in all parts of the jaw. The diphyodont s are 

 heterodonts, — i.e., the teeth are of various and 

 different forms in various parts of the jaw, — as 

 the incisors, canines, premolars, and molars of 

 the Mammalia. So as a rule homodonts develop 

 but one set of teeth, and heterodonts two sets, al- 

 though there are noted exceptions to this rule. 



In mammals the deciduous set arises de novo 

 from the mucous membrane, and the permanent 

 teeth are given off from the dental lamina. As 

 a rule each tooth of the first set is displaced ver- 

 tically by a similar tooth of the permanent set. 



