88 MAMMALIAN DENTITION 



other upper incisors in Macropus. The lower incisors becom- 

 ing elongated, lanceolate and more procumbent, provide, in 

 Macropus, a cutting edge which works against all three upper 

 teeth. The second upper incisors from their cramped position 

 may become very reduced. The third incisors tend to lengthen 

 antero-posteriorly perhaps because they have room to extend. 

 The canines, absent throughout in the mandible, become pro- 

 gressively reduced in the upper jaw until in Macropus merely a 

 small groove in the bone indicates their presence. The pre- 

 molars vary in number with age; in the young individual the 

 second and third are present in both jaws. As the animal grows 

 older these two are lost though the second remains longer in 

 Macropus than in the others for reasons to be specified later. 

 In each of the examples shown the second premolar is already 

 lost. In all but especially in the characteristically shoot-feed- 

 ing form, Dendrolagus, this tooth shows a well marked sectorial 

 character retained with some modification from the insectiv- 

 orous-omnivorous stage. 



The greatest changes are observable in the molars. The tooth 

 rows are slightly inturned posteriorly in all three types. The 

 molars of Potorous are short crowned quadritubercular buno- 

 dont teeth with slightly marked transverse ridges connecting 

 the two anterior and the two posterior cusps. There are also 

 ill-defined longitudinal ridges connecting the inner cusps in 

 the upper teeth and the outer cusps in the lower teeth. The 

 fourth upper molar shows a pseudohypocone produced as in 

 Phascolarctos by the division of the metacone. In Dendrola- 

 gus the teeth are still brachyodont but the transverse ridges 

 with the cusps have grown prominent to form distinct crests or 

 lophs. The antero-posterior ridge is also better marked. These 

 features are accentuated in the most advanced type, Macro- 

 pus, in which also the teeth are long crowned or hypsodont, the 

 increased length residing more in the cusps (lophs) themselves 

 than in the body of the teeth. This hypsodont character should 

 be contrasted with the hyposodonty in the bandicoot, Thala- 

 comys. The photographs of these three animals represent as 



