80 MAMMALIAN DENTITION 



Petaurus is an arboreal animal, its structure rendering it ill 

 adapted for terrestrial habits. Its diet consists partly of in- 

 sects and partly of the blossoms of the Eucalyptus tree. 



The tooth formula is usually given as: 



lf f C^,p|,.M-j.total40.' 



Z U o 4 



although the identification of the vestigial lower antemolar 

 teeth is difficult. Many evidences of considerable specializa- 

 tion are present. The elongated lower median incisor, the re- 

 duced upper canines, the irregularity of the upper premolars, 

 the vestigial character of the lower teeth between the median 

 incisors and the molars, the reduction in size from before 

 backwards of the molars themselves especially evinced in those 

 of the upper jaw, and the inturni-ng of the hinder ends of the 

 upper dental arch all indicate specialization. There are how- 

 ever certain primitive features which cannot be hidden. For 

 example the upper incisor tooth rows are directed toward 

 each other in typical insectivorous fashion. The median in- 

 cisors also are somewhat procumbent and have their tips ap- 

 proximated more than their bases. The third upper premolar 

 shows no sign of reduction. The median lower incisors are 

 procumbent and although elongated and greatly enlarged meet 

 the uppers in true forceps manner. Beyond these features the 

 dentition displays new traits for examination. The large size 

 of the median lower incisors has resulted in foreshortening of 

 the mandible and reduction or elimination of all the lower inter- 

 mediate teeth. The molars themselves no longer present needle- 

 like cusps or, in the case of the uppers, well marked styles so 

 characteristic of the insectivorous dentition. Their cusps are 

 rounded, mound-like or bunodont. The styles are practically 

 nonexistent and all trace of shearing action is lost. In the 

 upper teeth, in addition to the original proto-, para- and 

 metacones a fourth cusp appears behind the protocone and is 

 fairly well marked in all except the last tooth. This new cusp 

 is called the hypocone and its importance as a second pestle 



