OF THE AUSTEALIAN IMAESUPIALIA. 14.5 



in the Potoroinre and Macropodinse. As will be more apparent from a consideration of 

 the last-named division, the advent of herbivorous habits involves extensive v\-earing of 

 the molar crowns, which in the course of evolution is counteracted in part by a hypsodont 

 development and in part by a functional replacement of the teeth from behind forwards. 

 The anterior molars, since they come into service before the posterior ones, are naturally 

 worn down in the young animal, and if the teeth decreased in size backwards, as in the 

 Phalangerinae, the adult would be provided with a molar dentition totally inadequate for 

 its needs. Instead of this, the posterior molars increase at a greater rate than the 

 anterior ones, and also replace the latter when worn down, so that tlie animals are 

 provided with an efficient dentition up to an advanced age. In the Phalangerinte and 

 in those forms of the Macropodidas which have not yet advanced to a definite herbivorous 

 phase, the anterior molars may conveniently be larger than the others, because they 

 come into service at an earlier period and are functional throughout life. 



In Hypsip^ymnodou the molars are udiolly brachyodont, as in the Phalangerinse, in 

 Bettpngia their crowns are slightly elongated, and in JEpyprymnus considerably so. The 

 molar patterns of Bettongia present the same cliaracters as those of Sypslprymnodon. 

 In Mpyi)>'ymmis the increasing hypsodontism of the crowns causes the transverse ridges 

 to be more prominent than in the remaining forms. The posterior lobe in the fourth 

 upper molars shows indications of two cusps in all of the forms. The anterior lobe of the 

 tirst lower molars, both in Bettongia and JEpyprymnus, bears an accessory cusp on its 

 outer slope. The new cusp takes up the position of a protoconid, but is obviously 

 not homologous with that element, the true protoconid being the inner cusp, the true 

 metaconid having been lost in the phalangcrine stage. 



Incisors. — Throughout the series the median upper incisors are enlarged and elon- 

 gated. They are scarcely procumbent, and in this respect resemble those of the 

 Potoroinse and some species of Fseudochirus. The tips of the upper lateral teeth are 

 spatulate when unworn, and turned inwards so as to stop the passage of the lower teeth. 

 The latter are lanceolate. The general condition is little removed from that of the 

 advanced Phalangerinre. The special developments are few. Bettongia p)enicillata 

 practically repeats the characters of Ilypsiprymnodoii. B. Lestieuri shows a slight curving 

 of the median upper teeth with antcro-posterior compression of their tips, and also a 

 turning inwards of the anterior parts of the tips of the third teeth. All of these 

 characters are indicated in ^JEpyprymnus, and in a more marked degree. 



Canines. — These elements are fairly constant throughout, being greatly reduced but 

 not wholly vestigial. The canines are the last of the reduced elements to disappear in 

 the antemolar region. Their reduction is not, as in the case of the lower intermediate 

 teeth, attributable to the presence of the diprotodont moditication, since they are seen 

 to persist long after its introduction, but, as in ungulate Placentals, to the effects of 

 herbivorous evolution. 



