128 DE. B. A. BENSLET ON THE EVOLUTIOIV 



Uactylopsila represents the next liigber member of the series both in size and dental 

 characters. It differs from Petaurns in having the jirotocone and byjoocono fully 

 differentiated in the upper molars and placed on a level with the outer cusps, except in 

 the case of the first tooth, Avhere these elements are low as in the smaller species. The 

 external cusjjs do not, however, show quite so marked a tendency towards transverse 

 ridging as those of Petaitrus. Dacti/lojisila appears to 1)6 very slightly aberrant in this 

 cliaracter and in its insectivorous habit as compared with its size, judging from the 

 conditions presented by Fetaurus and more advanced forms. In the first molar tlie 

 jn'otoconid is low, as in JPetaurus. The remaining teeth have their cusps completely 

 bunoid. 



Witli Petaui'us and DactylopsUa the omnivorous development of the molars may 

 be regarded as complete. In the next member of the series, Phalanger, and also in the 

 iinal form, Ti'ichosurus, herliivorous modifications begin to be apparent. Tlie upper 

 teetli of the latter genus (PI. 5. fig. 16) are oblong in shape ; the hypocone is fully 

 developed, and the cusps are all of the same height. The internal band described for 

 Petaurus is now extremely conspicuous and involves completely the inner cusps. The 

 internal sides of the outer cusps are sharply ridged, each of the ridges extending from 

 the tip of an outer cusp towards the base of an inner one. In Phalanger this ridging is 

 slightly less prominent than in Trichosnrits. 



In the lower molars of Trichosunis (PI. 6. fig. lU) the same lophodont developments 

 are again in evidence, the opposite (internal) cusps being affected. The external band, 

 like the corresponding internal band of the upper teeth, is very pronounced and involves 

 completely the outer cusps. The first molar, both in Trichosurus and Phalanger 

 (PI. 6. fig. 21), shows the protoconid enlarged but low, as in Peiaurus and DactylopsUa ; 

 the metaconid is practically absent. 



The development of transverse crests in tlie final members of the Phalangerinse 

 represents the beginning of the herbivorous evolution. As will be seen from a 

 consideration of the Macropodidse and DiprotodontidaB, it is on these structures that the 

 herbivorous evolution of the molars is mainly developed. It is interesting to observe 

 the transverse ridges already appearing so far back in the evolution as Petaurus. 



Origin of Bunodont Molars. — On following the modifications of the molar patterns in 

 the Phalangerinae, it will be seen that the conditions obtaining in the final members 

 Phalanger and Trichosurtis represent a considerable advance on those of the initial 

 forms Acrobates, Pistcechurus, and Dromicia. Nevertheless, the molar patterns even of 

 the latter animals do not represent an actually incipient, but a faii"ly advanced phase 

 of the bunodont evolution, the first constructive stages having already been obliterated. 



The available evidence goes to show that the upjier molar patterns of the Phalangerinte 

 have been derived from a tiituberculate type like that seen in the Dasyurina3, the main 

 facts being as follows : — In the first jjlace, we find the earlier stage represented in the 

 group characterized by an imperfectly developed condition of the liypocone. This 

 appears to indicate that the hypocone is now in process of develojmient as a svipplement 

 to an originally trituberculate crown. The same differences in the degree of development 



