128 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. XIV. 



subrectangular tooth by the extension of the talonid and the antero- 

 posterior compression of the paraconid. These characteristics are seen 

 to a somewhat lesser extent in the Peramelidae and in general they lead 

 to conditions found among diprotodonts. The talonid still remains 

 slightly wider than the trigon, the individual cusps are still distinctly 

 elevated above the crests, and the molars as a whole are relatively long 

 and narrow. These appear to be primitive features and it is evident that 

 the lower molars are less advanced than the upper. Among the extinct 

 csenolestids, as noted by Sinclair, some are even slightly more primitive 

 in these respects than Ccenolestes while others show increased develop- 

 ment of transverse crests making the evidence quite clear for the 

 derivation of buno-lophodont types from tuberculo-sectorial. 



THE ORIGIN OF DIPROTODONTY. 



The diprotodont modification of the antemolar teeth has arisen in 

 so many diverse groups of mammals that it may well have had different 

 causes and courses of development in different cases. In some, however, 

 at least the general factors involved must have been the same. It occurs 

 in multituberculates, modern marsupials, insectivores, primitive eden- 

 tates, rodents, ungulates, bats and the lemuroids. In its most specialized 

 condition it consists in the complete suppression of all the antemolar 

 teeth except one pair of incisors in each jaw, these being functionally 

 modified and situated terminally. Among living marsupials, the pro- 

 gressive steps of this modification are illustrated by a series which is so 

 nearly complete that, except for the incipient stages, it could scarcely 

 be more convincing if it were in reality a linear phylogenetic series, 

 which of course it is not. Of living forms in which diprotodonty is 

 unmistakably developed, Ccenolestes undoubtedly has the most gen- 

 eralized dentition. From Ccenolestes to the more primitive phalangers, 

 as Dromicia and Distcechurus, the step is very slight; thence to the 

 herbivorous phalangers of the genus Trichosurus and on through 

 Hypsiprymnodon and Bettongia we come to the more typical macropods; 

 and in Phascolomys, although in other respects it is not a true terminal 

 member of this series, we see diprotodonty highly developed. This has 

 been traced in great detail and described by Bensley (1903) in the 

 important paper to which reference is so frequently necessary. Begin- 

 ning with Ccenolestes, the gradation from one form to another is almost 

 as complete as could be desired. 



The idea that Ccenolestes is not a true diprotodont, but has only 

 paralleled the diprotodonts in dentition, need not here be considered 

 at length, since it is only dentition that is important in this connection 



