158 DR. B. A. BENSLEY ON THE EVOLUrLOX 



PHASCOLOMYID.E. 



The dental cliaracters of Phascolomys, the single living representative of the family, 

 are of interest as indicating another line along which the herbivorous evolution begun 

 in the Phalangeridse has proceeded, and also as furnishing still another example of 

 convergent developments between Marsupials and Placentals, the general condition being 

 here very similar to that in the Rodentia. 



The exact relations of Phascolomys with the phalangerid genera are not wholly 

 demonstrable as regards the dentition. The main evidence is («) that the unworn molar 

 l^atterns present resemblance to those of the advanced bunodont Plialangerinse ; {b) the 

 incisor modifications represent a more advanced stage than is found in any of the latter 

 forms or in the Diprotodontidse ; {c) the moderately elaborated posterior premolars bear 

 a general resemblance to those of Dromicia on the one hand and, at least in the case of 

 the lower, to those of Nototherwm and Biprotodon. The animal appears to represent a 

 specialized otfshoot of the same line leading from the more primitive Phalangerinse to 

 the Diprotcdontidae. 



Molars. — In the upper molars of the adult Phascolomys the sectional area is seen to 

 decrease in proceeding from before backwards, while in tlie lower it is more uniform. 

 As in the more specialized members of the Rodentia, the teeth are elongated and open- 

 I'ooted. Their bodies are curved, outwards in the case of the upper teeth and inwards in 

 that of the lower ; so tliat the grinding-stress does not fall on the unsupported bases, 

 but on the side of an arch. Each tooth shows two triangular pillars; the sectional 

 apices are directed outwards in the lower teeth and inwards in the upper. In the 

 adult animal, the crown-surfaces being entirely worn down, the tips present the appear- 

 ance of two triangles joined together by their contiguous basal angles. In all of the 

 above characters, with the exception of the first, Phascolomys is not only highly 

 specialized, but also unique among existing Marsupials. 



The only safe evidence concerning the molar derivation is afforded by the unworn 

 patterns. The latter are represented in PI. 5. fig. 24 & PI. 6. fig. 30. Both the figures 

 and accompanying descriptions are based on a young specimen in which only three molars 

 above and below are formed, the first and second having barely pierced the gum, while 

 the third is still concealed in the alveolus. The upper molars bear a close resemblance 

 to those of Trichosurus (PI. 5. fig. 16). The two triangular pillars rej)resent the anterior 

 and posterior lobes of the phalangerine tooth. Each of the pillars bears two large cusps ; 

 those of the anterior one correspond almost exactly in their characters and position to 

 the protocone and paracone of Trichosurus, while those of the posterior pillar similarly 

 correspond to tlie hypocone and metacoue of that genus. As in Trichosurus, the cusps 

 are wholly bunoid, and the protocone and hypocone take part in the formation of a con- 

 spicuous internal band enclosing the internal anterior and posterior portions of the tooth. 

 The paracone and metacone are massive, and their internal sides show indications of the 

 development of transverse ridges. A peculiar feature is the presence of a large number 



