84 MAMMALIAN DENTITION 



on the upper and the occurrence of crescent-shaped cusps on 

 the lower molars. A feature of the mandible becoming more 

 marked with an increasingly herbivorous habit is the raising 

 of the condyle above the molar level with consequent increase 

 in the area of the ramus, reduction in breadth of the coronoid 

 process and clearer definition of the sigmoid notch. It is also 

 worthy of note that in the more purely herbivorous forms the 

 zygomata do not project so greatly beyond the brain case as 



i.XETB)C I^VST!* 



Fig. 24. — Dentition of the Koala (Phascolarctos cinereus, 9.222-2). This is an 

 example of the crescent-cusped molar pattern of the purely herbivorous dentition. The 

 third upper molar is an example of reversed evolution. 



in insectivorous and carnivorous forms. The upper incisors 

 are no longer arranged as for rasping but obviously as a coun- 

 tercutting edge for the elongated and procumbent loAver in- 

 cisors. In this function the median upper teeth alone are im- 

 portant, the second and third incisors being greatly reduced. 

 The upper canines are quite rudimentary; the lowers eliminated. 

 The single premolar in each jaw still shows some remnant of a 

 sectorial character. The molars, more especially those of the 

 upper jaw decrease in size from before backward. One of the 



