THE TEETH OF MAMMALS. 299 



of completeness ; the milk teeth in man answer the require- 

 ments of the child up to the age of seven years, and in the 

 Ungulata they commonly remain until the animal has 

 assumed its adult proportions. On the other hand, in 

 many " diphyodont " animals the milk teeth disappear very 

 early indeed, as in the mole (see Fig. 117) ; whilst there are 

 many instances of the milk teeth being absorbed in utero. 

 So that in the extent to which the milk teeth are developed, 

 the greatest variability is found to exist. 



A perfectly typical milk dentition represents, upon a 

 reduced scale, the adult dentition of the animal, with the 

 exception only that sexual differences are but feebly marked, 

 if indeed they are at all present. 



Thus, as a general rule, the' hindmost of the milk teeth 

 bear more resemblance to the true molars which come up 

 behind them, than they do to the premolars which come up 

 from below to displace them, which latter are generally of 

 simpler form. 



In what may be termed the normal arrangement, each 

 tooth of the milk series is vertically displaced by a tooth of 

 the permanent series ; but plenty of examples may be 

 found of particular milk teeth which have no successors, 

 and, on the contrary, individual permanent teeth which 

 have never had a deciduous predecessor. 



It has already been mentioned that amongst homodonts 

 no succession of teeth has been observed in the Cetacea, 

 nor in any other of the Edentata, save the armadillo ; 

 amongst heterodonts there are several Rodents which have 

 no deciduous teeth, e.g., the rat ; the dugong has probably 

 deciduous incisors, but no other milk teeth ; the elephant 

 has no vertical succession, save in the incisors. 



Among Marsupials, which are true heterodonts, there is 

 only one milk molar on each side in each jaw; this is always 

 displaced by the third or last premolar ; but the milk tooth 

 varies in the extent to which it is developed from being 



