OF THE AUSTKALIAN MAESUPIALIA. 173 



the projection is clue entirely to the elongation of tlie claw. In the skeleton examined 

 (text-tig. G, C) the fourth digit projects to ahout the same extent as the third. It is 

 nevertheless to be regarded as predominant. The metatarsal of the fourtli digit is 

 displaced backwards, so that the head of that of the third articulates with it as much 

 as with the conjoined trapezoid and cuboid. Its head is prolonged backwards beyond this 

 articulation, so that its tip extends to the margin of the foot, overla2:)ping that of the 

 enlarged fifth metatarsal. The fourth digit is tiius still predominant, l)ut has simply 

 l)een displaced backwards for better su2)port. It appears to be one of the niost functional 

 digits of the pes, and the fact that the sole is turned outwards and the hallux downwards 

 in action, the longest digit thus becoming subjected to great lateral stress, shows 

 sufRciently the reason of this adaptation. 



The available evidence is decidedly in favour of the derivation of the Notoryctes type 

 of pes from one of ordinary phalangerine type. The resemblance pointed out by Gadow 

 between Notoryctes and Didelphys is probably indicative of affinity, since, considering 

 the polyprotodont character of its dentition, Notoryctes must have been one of the first 

 derivatives of the arboreal syndactylous' line. The latter is directly traceable to the 

 DidelphyidfB. 



PHASCOLOMYID^. 



The pes of Phascoloinys (PI. 7. fig. 20) represents a derivative of the second 

 arboreal phase. The original plantigrade condition has been retained, as in Notoryctes 

 and Diprotodon. The hallux is reduced to a tubercle externally. In the skeleton 

 {cf. text-fig. 6, D) it occupies a position of opposability. The entocuneiform to which it 

 is attached is elongated, and its articular facet is external, so that the axis of the hallux 

 is placed at right angles to that of the second digit. The terminal phalanx of the 

 hallux has disappeared, and the proximal phalanx may also be absent. Externally there 

 is little indication of syndactylism of the second and third digits, the reason being that 

 all of the digits are more or less connected by integument. The condition is more 

 apparent in the immature specimen here illustrated than usually in the adult. In the 

 skeleton there is some variation in these digits. In some specimens they are very 

 distinctly slenderer than the fourth and fifth, while in others this disproportion is not 

 indicated. The fourth digit is always well develojied, as in the Phalangeridse. The 

 plantar surface shows no indications of the arboreal pads of the Phalaugeridie, the whole 

 of it being occupied by a tuberculate skin, as in Phascolarctus and Sarcophilus. 



DIPROTODONTID.E. 



In their original description of the foot-structure of Dlprotodon, Stirling and Zietz 

 (1899) show that it presents resemblances with the Phalangerid;e and the Phasco.- 

 lomyidae. Dollo (1899) has pointed out that the pes is of arboreal derivation, as 

 shown by the opposability of the hallux, the reduction of the second and third digits and 

 predominance of the fourth. The following summary, which is based partly on the 

 original description of Stirling and Zietz and partly on the plaster-casts recentlv 

 acquired by the British Museum, is designed to show that, as in the case of the 



21* 



