150 DE. B. A. BEXSLET ON THE EVOLUTION 



These "rooves are, however, so poorly marked that they do not produce the conspicuous 

 uotchinc^ of the edge seen in Fotorons. A noteworthy feature is the presence of a 

 postero-internal cusp. While indicated in Sypsiprymnodon and Bettongia, this element 

 is highly characteristic of the Macropodinse. The lower teeth are similar to the upper, 

 hut the accessory cusp is external. 



As in the Bettongiinse, the modifications of the median premolars follow those of the 

 posterior teeth, bvit represent a backward stage of development. Tiiese teeth are 

 unfortunately not indicated in any of the specimens of P. platyops and P. tridactylus. 

 In P. Gilberil they are decidedly more primitive than the posterior teeth, the upper ones 

 being comparatively small and broad, and bearing two notches like the posterior teeth 

 of P. platyops, while the lower ones show only one. In both of the Tasmanian species 

 the upper teeth show two notches, while the lower show two. In Caloprymnus they 

 show indications of two notches, above and below. 



A comparison of the most primitive member Potorous platyops of the present series 

 with the most primitive form of the Bettongiinae, Eypsiprymnodon moschahis, in respect 

 to the characters of the posterior premolars, will show sufficiently the necessity of 

 reo'arding the existing Macropodidge as of diphyletic origin. In the Bettongiinae the 

 premolars are well developed, conspicuously grooved, and their axes rotated outwards in 

 the most primitive of the known forms, and tbe occurrence of simdar modifications in 

 the allied form Burramys, which shows indications of phalangerine affinity iu its median 

 premolar, as in its mandibular characters, furnishes some evidence of the former 

 existence of the same peculiarity in a section of the Phalangerinte now without repre- 

 sentatives. The posterior premolars of Hypsiprymnodon resemble in a very general way 

 those of the more specialized forms of Flialanger. In the Potoroina; the posterior 

 premolars, in the most primitive condition known, are small and scarcely grooved, and 

 their axes are in the same line with the remaining teeth. We can only assume, there- 

 fore, that their characters have developed from somewhat similar ones in phalangerine 

 ancestors. The single-notched lower premolar of P. platyops is not far removed from 

 the type found in the upper teeth of I), lepida or the upper and lower in D. nana, in both 

 of which the tips are bifid. During the development of the Phalangeridse there has 

 been a development of sectorial premolars along at least four different lines, one of 

 these being indicated in the two genera Phalanger and Trichosurus, a second iu the 

 PhascolarctinsB, a third in the ancestors of the Bettoogiinse, and a fourth in Lroniicia 

 and the ancestors of the MacropodinaB and PotoroinaB. It is interesting to note that if 

 one may judge from the soaiewhat similar bifid condition in the less specialized forms 

 of Phalanger and in Dromicia, on the one hand, and the somewhat similar rotated and 

 o-rooved condition in the more specialized forms of Phalanger and in Kypsiprymnodon, 

 the first, third, and fourth lines are more closely related to one another than any one of 

 them is to the second. 



Macropodin^. 



In the case of the Potoroinae and Bettongiinae the dental evolution is of an e.xtremely 

 limited range, and for the most part of an indecisive kind. In the present series, on the 



