174 DR. B. A. BENSLET ON THE EVOLUTIOX 



dentition, there are general characters of resemhlance and special characters of diver- 

 gence bet\yeen the pes of Diprotodon and that of Phascolomijs, pointing to a common 

 origin of the two genera or the families they represent. 



Both in Biprotodon {cf. text-fig. 6, E) and Phascolomys the plantigrade condition is 

 retained. The hallux is reduced ; in the former only the metatarsal element is retained, 

 and its distal portion is swollen to form a knob-like structure. Although reduced in 

 respect to the loss of plialanges, this digit was apparently functional in Biprotodon, 

 serving as an antero-interual support, the remaining antero-internal elements, especially 

 the second and third digits, being poorly developed. In both forms the entocuneiform is 

 elongated and its articular facet (as opposed to terminal) is external, as in the Phalan- 

 geridae ; so that the hallux is set away from the remaining digits. The two forms differ in 

 the relations of the mesocuneiform element. This is free in Phascolomys and joined to 

 the entocuneiform in Diprotodon. In both forms the second and third digits are dis- 

 proportionately smaller than the fourth. Dollo has correctly pointed out that the larger 

 size of the fourth digit in Diprotodon is partly inherited and j^artly adaptive. In Phasco- 

 lomys the second and third digits have regained their functional importance, while in 

 Biprotodon, they have not. In the former the walking-stress is uniformly distributed 

 to all of the digits ; in Biprotodon it must have fallen on the outer side of the foot, only 

 the hallucal metatarsal, beyond the more posterior navicular and calcaneal protuberances, 

 serving for support internally. This is apparent not only in the general disjiroportion 

 between the fourth and fifth digits on the one hand, and the second and third on the 

 other, but also in the massiveness of the astragalus, with its internally directed facet, 

 calcaneum, cuboideum, and fifth metatarsal. The shifting outwards of the walking-axis 

 is attributable in the first place to the reduction of the opposable hallux common to all 

 terrestrial forms, and in the second to the reduction of the second and third digits as a 

 phalangerine arboreal adaptation, the three internal digits being thus largely thrown out 

 of service. The divergent characters presented by Phascolomys and Biprotodon are 

 explainable on the assumption that in the former the second and third digits have 

 regained their functional importance, and that in the latter the hallux has remained 

 functional to a slight extent by being modified as a prop or internal balancer. There is 

 little doubt that the two forms are divergent members of a single terrestrial plantigrade 

 line leading from the Phalangeridse. 



PERAMELIDyE. 



The general type of pes in the Peramelidse agrees with that of the three preceding 

 families in that it represents a derivative of the second arboreal phase. It differs, 

 however, in the substitution of digitigrade developments for plantigrade ones, with a 

 •tendency towards functional monodactylism. 



The Peramelidse are without existing arboreal representatives. The only way in which 

 it is possible to arrange their foot-patterns in proper sequence is, therefore, according 

 to the extent to Avhich they depart from the arboreal type as presented by the 

 Phalangeridse. In such an arrangement the most primitive conditions are seen to be 

 presented by the two forms Perameles Haffrayana and P. Cockerelli. It is an interesting 



