OF THE AUSTEALIAN MAKSUPIALIA. 1^5 



Ccenolestes the fourth up])er incisor shows a tendency towards reduction, while in 

 Disfa'ch/triis, as, in fact, in all of the Phalangeridse, the number of u[)per incisors is 

 already reduced to three, the lateral two together serving as a stop for the enlarged 

 lower teeth. 



The presence of the diprotodont modification in herbivorous Marsupials is accordingly 

 not to be explained on the supposition that it represents an advantageous condition, but 

 rather that these animals are the descendants of minvite forms, Avhich, like Acrohates, 

 Disla'chunis, and Dromicia, had undergone a special insectivorous development of the 

 median lower incisors, and that their ancestors were able to remodel the original 

 insectivorous condition to suit the necessities of an herbivorous evolution. In the 

 Macropodidge both the upper and lower incisors present trenchant modifications 

 adapting them more or less perfectly for a grazing habit ; while in the Phascolorayidre 

 and Diprotodontidse (PI. 5. figs. 40, 41) the median upper and lower teeth present an 

 open-rooted condition, and a formation of anterior enamel bands fitting them for a 

 rodent habit. 



Aj^art from its bearing on the evolution of the Phnlangeridte, the origin of the 

 diprotodont modification as an insectivorous modification, primarily concerning the 

 median vipper and louer incisors, and its conversion into a rodent modification in 

 the Phascolomyidae, is of interest as furnishing a possible clue to the origin of the 

 placental Rodentia. 



With I'egard to the special dental evolution of the Phalangeridte, we may distinguish 

 three lines of development, characterized by the modifications of the molar patterns as 

 follows : — 



A. Molars quadrituberculate, with selenoid cusps. Upper teeth with reduced 



e.\.terual styles ; with or without intermediate conules Pliascolarctinse. 



Genera : Pseudocliirus, Petauroides, Phascolurctu.s. 



B. Molars quadrituberculate, with bunoid cusps. Upper teeth without external 



styles or intermediate conules Phalang-erinse. 



Genera : Acrubates, Distwchurus, Drvmicia, Gymuobelideus, 

 Petaurus, Dactylopsila, Phalanger, Trichosurux . 



C. Dentition degenerate ; cheek-teeth haplodont, wholly vestigial Tarsipedlnse. 



Genus : Tarsipes. 



PHALANGERINiE. 



As compared with the Phascolarctinae and Tarsipedinse the present group will be seen 

 to represent the dominating division of the family, the bunodont modifications of the 

 molars by which it is distinguished being found in eight of a total number of twelve 

 genera. It also represents the ancestral group from which the herbivorous families 

 have been derived. 



The various genera of the Phalangerinte form a progressive series of nmch the 

 same order as that seen in the case of the Dasyurintc, the dominating principle apjjcaring 

 to be, as before, the increase in size of the body. The more primitive forms, such as 

 Acrobates, Distoechuriis, and JJromicia, in their comparatively minute proportions and, 



18* 



