LIFE HABITS AND DENTITION 81 



accessory to the protocone becomes evident. There is a slight 

 ridge connecting the proto- and hypocones and indications are 

 present also of ridges passing transversely from the proto- 

 and hypocones to the para- and metacones respectively. 

 Finally from the hypocone a ridge ill-marked as yet passes 

 round the anterior margin of the tooth in front of the paracone 

 and a similar crest originating in the hypocone runs round the 

 posterior margin behind the metacone. All these features are 

 deeply significant as forecasting the commencing herbivorous 

 adaptation. 



In the mandible the molars, excepting the first, are quadri- 

 lateral in shape and display four bunodont cusps, the proto- 

 conid, metaconid, entoconid and hypoconid. The paraconid is 

 eliminated. There is a slight external ridge connecting the 

 proto- and hypoconids. In the first molar not only is the para- 

 conid absent but the metaconid is vestigial and the protoconid 

 shows an appearance reminiscent of its relatively great devel- 

 opment in insectivorous forms since it exhibits to a slight de- 

 gree a sectorial character. These bunodont molars are very 

 serviceable in triturating food for the pestle and mortar rela- 

 tionship of the protocone and talonid is aided by a similar in- 

 teraction of the hypocone with the anterior slope in front of 

 the protoconid and metaconid of the lower molar next behind. 



All the Marsupials hitherto described have belonged to the 

 suborder Polyprotodonta, so-called because they possess more 

 than one functional incisor on each side of the mandible. 

 Petaurus is the first example of the suborder Diprotodonta, the 

 members of which possess only a single pair of functional in- 

 cisors in the lower jaw. Referring then once again to the den- 

 tition of Petaurus it is seen that the second and third upper 

 incisors merely act as stops to the long lower incisors. It will 

 shortly be apparent that the dentition of the herbivorous Dipro- 

 todonts is developed directly from the insectivorous type 

 through the omnivorous adaptation by a peculiar elongation 

 of the median lower incisor. There is however no direct line 

 of evolution in which all the steps may be folloAved. Hence it 



