134 DE. B. A. BENSLET ON THE EVOLUTION 



The modifications of the posterior premolars in the Phalangeringe will be seen to 

 furnish a further example of the conflict of developments already described for the 

 incisors. In Acrohates and Distoechurus these teeth present two stages of a reduction 

 similar to that which is so characteristic of the Dasyurinfe. Within tlie limits of tlie three 

 species of Dromicia, we find the same tendency towards reduction indicated in D. concinna, 

 but an opposite tendency towards enlargement as sectorials in D. lepida, and still more 

 in D. nana. In Fetaurm and Dactnlopsila the posterior premolars are comparatively 

 poorly developed, this being especially the case with the lower testh, while in the 

 succeeding forms Fhalanger and Tnchosurus they are greatly developed as sectorials. 

 Tlie marked diff'erence in this respect between Fhalanger and Dactylopsila is doubtless 

 due in part to the retention in the latter of insectivorous modifications of the incisors, 

 the latter teeth being thus the more functional elements; hut, in general, the modifica- 

 tions of the whole series represent a conflict between a reduction of the posterior teeth, 

 as in the Dasyurina?, and their elaboration as sectorials, as in the Macropodidae. 



Lower intermediate Teeth. — The functional or vestigial character of the various 

 teeth situate between the median lower lower incisors and the first molars in this series 

 probably depends on several circumstances, the elongation of the median incisors being, 

 however, the most important factor in the reduction of the elements of this region. 

 In all forms, except Trichosurus and Fhalanger, there are normally two vestigial teeth 

 situate immediately behind the median functional ones. They are sometimes separated 

 .from the median premolars by diastemata which, in all probability, were formerly 

 occupied by the canines and anterior premolars. The vestigial nature of these teeth 

 can only be ascribed to the enlargement of the median incisors. The latter would 

 doubtless have brought about the reduction of the median and posterior premolars in 

 all forms, as in Petaifnis and Dactylopsila, were it not for the fact that it has been 

 prevented from so doing by other developments. Thus, in Acrohates and Bistoechtirus 

 the median premolars have been prevented from becoming vestigial by being developed 

 as piercing organs. In Fhalanger, Trichosurus, and Dromicia nana the posterior teeth 

 have been prevented from hecoming vestigial by being developed as sectorials. 



The only feature of phylogenetic interest presented by the intermediate teeth concerns 

 their relations in Fhalanger and Trichosurus. In the former three vestigial teeth are 

 usually, although by no means always, present*, while in Trichosurus only one is as a 

 rule indicated, although two may occur as a variation. This fact supports the evidence 

 already given as to the more advanced position of the latter genus. 



Taesipedin.^:. 



The single representative of this division, Tarsipes rostratus, owes its distinction from 

 the remaining members of the family to the wholly degenerate character of its dentition, 

 the only teeth represented being the upper incisors, the median lower incisors, the upper 



* Bateson (1894, p. 253) has given a detailed account of the variability of these disappearing elements in 

 P7ialanr/er and other members of the present family. 



