138 DE. B, A. BENSLEY ON THE EVOLUTION 



usually more prominent than the hypocone from an inward extension of its base. This 

 condition appears to be a remnant of that seen in more primitive trituberculate forms. 

 The spaces between the cusps present a curious appearance, the surface being throw n 

 into small ridges. The posterior shearing edge of the jiaracone is frequently supple- 

 mented by a larger ridge passing down its postero-internal side. The external styles are 

 almost as conspicuous as in Pseudochirus, not, however, on accoiuit of their better 

 development, but from the greater concavity of the outer cusps ; they are in reality 

 more vestigial than in the latter genus. Style b does not project beyond the level of the 

 paracone ridge which joins it ; style c is very minute and bifid. 



In the fourth upper molar the posterior lobe bears two perfectly developed selenoid 

 cusps, so that the tooth is almost comjiletely quadrate like the others. As already 

 mentioned above, this represents the final stage of a process of cleavage of an originally 

 single cusp. 



The lower molars of Phascolarctus (PI. 6. fig. 23) resemble those of Fseudochirus even 

 more closely than do the upper. The selenoid character is more pronounced in tlie outer 

 than in the inner cusps. The latter are, however, more selenoid than in Pseudochirus, 

 a slight concavity having now appeared on their inner faces. These teeth show a 

 tendency toAvards the addition of an external cingular ridge, obliterating the notch 

 o}'iginally present between the protoconid and hypoconid. 



Origin of Selenodont Molars. — Like those of the Phalangerinte, the molars of the 

 present division are comparatively well specialized. Apart from the more or less 

 completely selenoid modifications of the cusps, the hypocone will be seen to be well 

 developed in the upper teeth, Avhile the paraconid and hypoconulid are wholly absent in 

 the lower. Tliey are thus considerably removed from the secodont tyj)e characteristic of 

 the polyprotodont section. The question here arises as to the intermediate stages which 

 have been passed in the development of the selenodout molars up to the Pseudochirus 

 stage. 



It may be admitted at the outset that the evidence on this subject is of a rather 

 fragmentary kind. There is, in the first place, no marsupial group outside of the 

 Phascolarctinte which shows incipient stages of the same evolution, as Caluromys does 

 with reference to the Phalangerinse, and, in the second place, the evidence as to the 

 origin of the selenodont molars in the j^araliel case of the artiodactyl Ungulata is not 

 wholly satisfactory *. 



Winge has expressed the opinion tliat the molar patterns of Phascolarctus arc 

 ancestral to those of the Phalangerinse, intermediate conditions being found in Pseudo- 

 chirus. The main reason advanced for this view is that Phascolarctus shows the 

 same angular character of the molar cusps as is found in the polyprotodont forms, 

 and further that it possesses vestigial external styles, which have disappeared in the 

 Phalangerinte. 



Three propositions may be considered, namely : («) that the molar patterns of the 

 Phascolarctinse are ancestral to those of the bunodont Phalangeriniie ; {b) that the reverse 



* The available cvideDOO favours buuodoiit origin of selenodont (artiodactyl) types. 



