OF THE AUSTRALIAN MAESL'PIALIA. 143 



Generally speaking, the dental evolution of the Macropodidee has been connected 

 Avith the perfection of the dentition for a grazing habit, and this, in turn, appears to 

 have been closely associated with increasing capabilities of the animals for terrestrial 

 progression. Judging from the sequence now presented, the ancestors of the family 

 possessed a type of dentition fully as primitive as that of Petaiirus, and their sphere 

 of action must have been greatly limited by their small size and the shortness of their 

 limbs. Taking these phalangerine characters into consideration, it is probable that the 

 original forms lived among the low scrub, subsisting on leaves and shoots, and possibly 

 supplementing this fare Avitli roots and grasses. The most specialized members of the 

 family, namely, the Kangaroos, judging from the present characters and habits of the 

 smaller forms, are the descendants of such phalangerine forms, which, on becoming larger 

 and longer limbed, and from browsing on the herbage of the scrub, have taken to grazing 

 in its open spaces, then to the open forest-glades, and finally to the open plains. 



In ascribing to the MacropodidjB tlie perfection of grazing adaptations as the chief 

 feature of their dental evolution, a few reservations must be made. The most primitive 

 members of the existing family (Eettongiinse, Potnroinae) are forms which have not taken 

 part in the grazing evolution, the direction of their special evokition being, with one 

 exception, away from rather than towards the typical forms. Furthermore, it seems 

 probable that certain of the extinct forms, such as Frocoptodoti, judging from the 

 characters of the incisors, possessed feeding-habits of a different kind from those of their 

 grazing contemporaries. Finally, certain of the more advanced existing members of the 

 lamily, embracing the Tree- and Dorca-Kangaroos, from being terrestrial, have become 

 temporarily or permanently arboreal, and while retaining many primitive characters have 

 undergone a divergent evolution of sectorial premolars as a result of shoot-eating habits. 



The various existing genera of the family may be arranged, on a basis of their premolar 

 and molar characters, as follows : — 



A. Molars lopliodont, their anterior and posterior pairs of cusps completely connected 



by transverse crests Macropodinse. 



(a) Molars brachyodont : sectorial premolars excessively developed. 



Genera : Denilrolagus, Dorcopsis, Setonyx. 

 {b) Molars hypsodout ; sectorial premolars moderately developed. 



Genera: Lar/orchesles, Lagostrophiis, Onijchogale, Petrogale, Macropus. 



B. Molars quadrituberculate, their cusps only incipiently lophoid. 



[a] Sectorial premolars with from two to four short superficial grooves . . . Potoroinse. 



Genera : Potorous, Caloprymnus. 

 (/;) Sectorial premolars with six or more long, narrow, and prominent grooves . Bettongriinse. 



Genera : Hy psiprymnoduit, Betlongia, zEpyprymnus. 



Bettongiin^, 



The three genera Hypsiin'ymnodon, Bettongia, and u-Epyprymnns, which are included 

 in this division, owe their distinction to the possession of well-differentiated sectorial 

 premolars associated with primitive characters in the remaining teeth. They present a 

 line of dental evolution entirely distinct from that beginning in the PotoroinjB and 



