LIFE HABITS AND DENTITION 89 



nearly as possible individuals at corresponding stages of life. 

 But whereas in Potorous and in Dendrolagus all the molars are 

 erupted and the first worn only slightly more than the fourth 

 in Macropus the fourth is still not fully erupted although the 

 first is greatly worn. A similar condition will be noted in the 

 grazing Ungulates and in the Elephants. The grazing habit 

 causes greater wear of teeth than any other form of feeding and 

 Nature, in defense, has arranged that the hinder molars erupt 

 later in these than in other animals and come into action as the 

 more anterior teeth wear out. Several other features of the 

 dentition of the Kangaroos are explicable upon the same basis. 

 The hypsodont character of the teeth, the transformation of a 

 bunodont into a lophodont dentition and the exaggeration of 

 the longitudinal ridges are examples. Again whereas in Poto- 

 rous the last molar is the smallest, in Macropus this tooth has 

 become secondarily enlarged, an example of reversed evolu- 

 tion. In this respect Dendrolagus represents an intermediate 

 stage. As the smaller but more numerous anterior teeth be- 

 come worn and fall out the longer hinder teeth erupt, migrate 

 somewhat forward in the jaw, and provide the animal with an 

 unshortened molar row. The reasons for retention in earlier 

 life of the second premolar in addition to the third is now clear. 

 This tooth assists the molars before the eruption of the hinder 

 teeth. Lastly the herbivorous Marsupials differ from insec- 

 tivorous and carnivorous forms in presenting a better marked 

 vestige of the milk dentition. All herbivorous Marsupials have 

 a well-developed milk molar which remains in position a com- 

 paratively long time before it is replaced by the sectorial last 

 premolar. The milk tooth thus assists the first permanent 

 molar in function and only falls out when much worn. In 

 Macropus, counting the milk molar, there are seven cheek 

 teeth (two premolars, four permanent molars) of which, how- 

 ever, only three are present in old animals. 



The tooth formula for the Kangaroos may now be stated but 

 it must be remembered that these formulae vary with age. 



