196 DK. B. A. BENSLET 0>^ THE EVOLUTIOIS- 



of the pes, Avliicli shows on tlie whole little departure from the phalangerine type. Both 

 species are specialized in the reduction of tlie upper incisor formula and iu the reduction 

 of the plantar pads. P. CockerelU is more specialized tlian P. Boreyana in its molar 

 patterns. P. lia-ffrayana is not far removed from the stem-form of all the Australian 

 mainland species, as shown by its retention of a full upper incisor formula, in common 

 with all of the latter, retention of prototypal characters in the molars, retention of more 

 primitive proportions of the pes and of plantar pads. 



The evidence of the dentition and foot-structure will be seen to favour the view of a 

 Papuan origin of the family, the three forms P. Boreyana, P. Baffmyana, and P. Cocherelli, 

 possessing the most primitive characters, being confined to New Guinea, while all the 

 advanced forms are Australian. 



Thomas (1888) has remarked that the Australian species fall into two series typ)ified by 

 P. obesula and P. Giouii respectively. These divisions indicate two lines of development, 

 giving rise respectively to short- and long-footed types. Tiie former includes P. obesula, 

 with its dwarfed relative P. barroiceusis, and P. macruru, the latter the remaining species 

 ol Terameles, together with Thylacomys and Choerojnis. The short-footed forms are, 

 speaking generally, also distinguished by the retention of the original shortened condition 

 of the muzzle, in wliich character they are even more primitive than P. Raffrayana, and 

 by the possession of highly specialized molar patterns, the latter being of the most 

 advanced type found in the family. 



Assuming a Papuan origin of the family, the question arises as to whether the 

 Australian mainland species are derivatives of a single stem-form or are diphyletic. 

 This question is not easy to decide because of the lack of adherence to type which is so 

 conspicuous a feature of the whole family. The Papuan species P. moresbyensis is, as 

 pointed out by Thomas (1888), closely allied to the North-Australian P. macrura. It 

 represents a very slight advance on P. Rajfrayana in foot-structure, and a very decided 

 advance in its molar patterns, the latter being almost as specialized as those of P. obesula. 

 F. macrura is not, as might be expected, prototypal to the southern and Tasinanian 

 P. obesula, but is more specialized both in dentition and foot-structure. The long-footed 

 series must be traced to a form combining the dental characters of P. Bougainvillei with 

 the pedal characters of P. nasuta. The presence in New Guinea of P. moresbyensis with 

 affinities with P. macrura and P. obesula appears at first sight to indicate tliat there lias 

 been a differentiation in New Guinea of two stem-forms, one combining a primitive type 

 of dentition with a slightly elongated type of pes, the other combining a specialized type 

 of dentition witli retention of the original shortened type of pes. It is much more 

 probable, however, that P. moresbyensis has arisen independently of the Australian 

 forms P. macrura and P. obesula, and has become specialized in dentition. If this is 

 the case, it bears the same relation to P. Raffrayana as P. CockerelU does to P. Boreyana. 

 P. CockerelU, it will be observed, is also independently specialized in its molar patterns. 

 There is a remote possibility that the presence of P. moresbyensis iu New Guinea 



is secondary. 



Of the mainland species P. macrura may be safely regarded as a more specialized 

 offshoot of the same form which gave rise to P. obesula. The distribution of the two 



