132 DE. B. A. BEXSLET OX THE EYOLUTIOX 



cliaracters. Those of D. lepidu are of interest as being prototypal to those of Fetaunis. 

 lu this species the upj^er anterior and median teetli are small as compared with the 

 2DOsterior ones, the latter, unlike those of Acrohates and Distcechurus, being enlarged 

 instead of reduced. They present a rather different appearance from those of Acrobates, 

 being relatively short and provided with accessory cusps. The tips of the upper 

 posterior premolars are slightly flattened and bifid. Of the lower premolars the only 

 functional members are the posterior teeth, which are comparatively well developed 

 and also bifid. In D. concinua the disproportion in size between the upper anterior 

 and median and the posterior premolars is almost more marked than in D. lepida, 

 not, however, on account of the enlargement of the latter, these being in reality less 

 enlarged, but because of the styliform and wholly vestigial character of the former. 

 In the lower jaw of this si^ecies none of the premolars are functional. In D. nana, 

 a larger and more sjoecialized form than the others, the anterior and median upper 

 joremolars are vestigial, as in D. concinna, while the posterior teeth are greatly 

 enlarged, and much more completely bifid than those of D. lepida. As in the latter, 

 only the posterior premolars are functional in the lower jaw ; they are enlarged bat 

 not bifid. 



As already mentioned, Petaurus approximates to D. lepida. This is especially true of 

 P. breviceps, the smallest of the three species. The upjier anterior and median premolars 

 are low, with prominent accessory cusps. The median teeth, which in D. lepida are 

 reduced as compared with those of Acrobates and Distcechurus, are here further 

 reduced. Tlie upper posterior premolars are moderately developed; they are not bifid 

 at the tips, although they show a tendency tow^ards such a development in a grooving of 

 their outer sides in the region of the edge. The lower posterior premolars are reduced ; 

 they show a faint tendency towards a grooving of the edge. It appears probable that 

 both upper and lower teeth have been formerly modified as in I), lepida. 



Dactylopsila shows an advance on JPetanrus, except as regards the upper anterior 

 premolars. The latter are seen to be undergoing a new development, becoming less 

 compressed, single-rooted, and subcaniniform. Apart from its striking character, this 

 development is of great interest as being carried over from the pi-esent form to the 

 succeeding genus Phalange)'. The median premolars are still more reduced than in 

 Petanrus, being -s^ holly vestigial or even absent. The posterior teeth are moderately 

 developed, and their edges are slightly compressed in an oblique direction, the anterior 

 portion of the edges being placed internally. This is the reverse of the condition in 

 Phalanger, where the edges are compressed in such a way that the anterior portions are 

 external. None of the lower premolars ai'e functional in Dactylopsila ; the posterior 

 teeth are still more reduced tlian in Petaurus. 



In the succeeding genus Phalanger the anterior and median upper premolars show 

 the same trend of evolution as those of Dactylopsila, while the posterior teeth above and 

 below show a pronounced departure. Of the former teeth the anterior elements are 

 single-rooted and subcaniniform, except in Phalanger ursinus, ■while the median ones 

 are vestigial or wholly absent. The posterior premolars are enlarged as sectorials, as in 

 Dromicia nana. The various steps in the elaboration of the sectorials are illustrated to 

 a certain extent in the different species. In the upper teeth of P. celebensis, P. breviceps, 



