OF THE AUSTEALIAN MAESUPIALIA.. 



123 



Considered alone, the above cliaractevs might be regarded as indicating that the dental 

 evolution of the Phalangerida? represents a continuation of that of the Peramelidae. That 

 this is not the case, ho^vever, will be seen from the following additional characters : — 

 (^0 The primitive members of the Phalangeridge retain a differentiated (insectivorous) 

 condition of the median upper incisoi's. This is not indicated in any of the Peramelidte, 

 although present in all other primitive polyjn'otodonts (Dasyurid8e,Didelphyid£e). (b) The 

 same forms show signs of dasyurid affinity in the retention of normal insectivorous 

 characters in the upper canines and 2^remolars, in which respect they are quite as 

 primitive, and, in view of their smaller size and the tendency towards retrogression of 

 these teeth, especially the canines, in the Peramelida3, even more so than the latter familj'. 

 ((?) The primitive members of both families present an incomplete condition of the 

 hypocone in the upper molars, Avhile tlie advanced members in each case show this 

 element in a j^erfeet condition, (d) Finally, the Phalaugeridae show none of those 

 hypsodont developments of the molars which characterize the other family. 



The explanation of tliese two series of characters becomes more apparent on comparison 

 of the foot-structure in the two families. The present facts may be taken as indicating 

 that the common ancestors of the Peramelidse and Phalaugeridae were in all probability 

 animals combining the incipient omnivorous molar characters of tlie primitive members 

 of the former family, and also their incisor formula, with the normal antemolar characters 

 of the smaller Dasyurinoe, and it may be mentioned in anticipation that the two families 

 have undergone a divero'ent evolution in the foot-structure — the Peramelida? liavinc; 

 become terrestrial, Avhile the Phalangeridfe have remained arboreal, ,so that their 

 omnivorous dental evolution must have proceeded independently as far as the later 

 stasres are concerned. 



T/ie Origin of f lie Diprotodont Jlodificatiou. 



Closely connected with the question of the dental relations of the Phalangeridic and 

 Peramelidtie is that of the origin of the diprotodont modiiication, which appears for the 

 first time in the former familv. althou2:li characteristic to a marked degree of the whole 

 herbivorous section. 



It is a familiar fact that of the recent Marsupials the Didelphyida^ Dasyuridie, and 

 Peraraelidoe on the one hand, and the Phalangerida;, Macropodid*, Phascolomyida;, and 

 Diprotodontidaj on the other, form two broad divisions, diffeiiug markedly in the 

 characters of the antemolar teeth, which distinction led Owen to propose a p.imary 

 division of the wliole group into Polyprotodontia and Diprotodontia. Tlie relative 

 merits of this division will be discussed in a subsequent section, the object of the 

 following remarks being to show that the diiirotodont modification, although charac- 

 teristic of the herbivorous section of the Marsuinals, is the result of an insectivorous 

 adaptation which must have been developed in the mi luite ancestors of thePhalangeridu.- 

 during the incipient stages of the omnivorous evolution, but after the separation of the 

 peramelid stem. 



The fundamental characters of the diprotodonts maybe enumerated as follows : — 

 {it) Pi,eduction of the upper incisor formula to 3 or even 1. (6) Pieduction of the lower 



SECOND SERIES. — ZOOLOGY, VOL. IX. 18 



