TEETH 



249 



The limitation of the succession to two or even one functional 

 set is probably due to the concentration of several successive 

 generations of teeth in correspondence with the higher develop- 

 ment of the individual tooth. This concentration is most marked 

 in Marsupials, in which only a single tooth, usually described as the 

 fourth premolar, has a predecessor. Differences of opinion exist as 

 to whether this tooth is to be regarded as the last remains of the 

 first or of the second set, or whether it belongs to the same series 

 as the others and is only retarded in development. The fact that 



FIG. 200. DENTITION OF A CATARRHINE MONKEY (Nasalis larvatus). 

 References as before. 



in toothed Whales the milk-teeth persist and the second set is 

 only represented in rudiment, seems to indicate that the teeth of 

 Marsupials, except the fourth " prernolar " belong to the first set, 

 and that the milk dentition of Mammals is not a secondary acqui- 

 sition. In other words, the primitive Mammalia were at least 

 diphyodont, and the apparent monophyodont condition seen (e.g., in 

 toothed Whales) is a secondary condition. In the placental Mammals 

 the second dentition becomes of greater importance than the first. 1 



1 In many instances, however, the first premolar appears to belong to the 

 milk dentition, and this may possibly be the case as regards the molars also. 



