MAY, 1921. AMERICAN MARSUPIAL, CJENOLESTES OSGOOD. 121 



one-sixth as large as the third, and it is turned slightly inward from the 

 toothrow. It is roughly triangular and carries three small cusps appar- 

 ently representing paracone, protocone, and metacone. They are 

 scarcely higher than the commissures which connect them and inclose a 

 central depression. There is still retained an external cingulum, a 

 minute anterior shelf, and two roots, one of which is larger and obviously 

 formed by coalescence. This tooth is plainly undergoing reduction and 

 is in about the same stage as the fourth molar of Dromicia. In Acrobates 

 and Distaechurus it has quite disappeared. 



Discussion. The upper molars of Ccenolestes show an obvious resem- 

 blance to those of the diprotodont family Phalangeridae. Two principal 

 types of molars are found in this family: (i) the bunoid and (2) the 

 selenoid, respectively characterizing the two subfamilies Phalangerinae 

 and Phascolarctinae. In general, the bunoid group are without styles 

 or intermediate conules while the selenoid group usually have at least 

 traces of both styles and conules, but in these respects there are excep- 

 tions in both groups. 1 Several theories have been advanced as to the 

 origin and mutual relationships of the two groups. Winge considered 

 them derivative the one from the other and regarded Phascolarctos 

 ancestral to the bunoid Phalangerinae with Pseudochirus as a connecting 

 form. On the other hand, students of placental molars have usually 

 inclined to the belief that, among ungulates especially, bunoid types were 

 ancestral to selenoid. Bensley, however, comes to the conclusion that 

 "the molars of the Phascolarctinae and Phalangerinae have been de- 

 rived by a divergent evolution from a common insectivorous secodont 

 type," that is, independently from a tritubercular or post-tritubercular 

 type similar to that seen in the Peramelidae. This divergent evolution 

 took place, according to Bensley, as follows : 



1. The selenoid type evolved by the simple addition of a hypocone 

 and the reduction of the styles, no change in the shape of the principal 

 cusps being required since they are already selenoid in the tritubercular 

 molar. This method was recognized also by Winge. For the production 

 of the molar of Pseudochirus the addition of the intermediate conules was 

 further required and that these were secondary elements which arose 

 de novo seems to have been taken for granted, although, as shown later, 

 there is some evidence that they are not. 



2. The bunoid type evolved also by reduction of styles but accom- 

 panied by a lowering of the principal cusps and a gradual change in their 

 form from crescentic to subconical. These and other changes involved 



1 In the Phalangerinae, traces of a metaconule occur in Petaurus and a large 

 persistent style is present in Dactylopsila. On the other hand, the Phascolarctinae 

 include some species of Pseudochirus in which styles are lost and Phascolarctos in 

 which conules are scarcely evident. 



