THE DECIDUOUS DENTITION 253 



dentition is heterodont, whereas the permanent teeth are homo- 

 dont. In the toothed Whales the milk teeth are entirely vesti- 

 gial. Herbivorous feeders on the other hand retain at least 

 some of their milk teeth until they are sufficiently adult them- 

 selves to give birth to young and this is probably an adapta- 

 tion to save the permanent teeth in view of the extreme attri- 

 tion to which, in consequence of the nature of the food, the 

 dentition of these animals is subjected. 



Fig. 93. — Deciduous dentition of Dog (Canis familiaris, 9.7424-47). The milk denti- 

 tion as a whole reproduces the features of the permanent dentition as a whole. The 

 large simple tooth erupting immediately behind each canine is the first premolar of the 

 permanent set, a tooth which in most animals does not possess a predecessor (but see 

 Tapir, Fig. 77). 



According to some authors there are evidences of as many 

 as five dentitions among Mammalia, one before the milk teeth, 

 one after the permanent set and another antecedent to the 

 premilk series. This last rests, it must be admitted, upon very 

 slender evidence. Dr. Ameghino is responsible for the state- 

 ment that in a long extinct Ungulate, Nesodon, there were de- 

 veloped three successive dentitions of which the second and 

 third correspond with the modern milk and permanent series 

 respectively. The same author asserted that at least one func- 



