130 DE. B. A. BENSLEY OX THE EVOLUTION 



The lav^er forms Tetaiirns and Dactylopslla show a more advanced stage of the same 

 modification. In the former the median ujiper incisors are decidedly procumbent. 

 They tend to he trianguLnr in section from a thickening of their antero-interaal edges. 

 In Dactylopsila they are still more completely triangular in section, and the procumbent 

 character is very conspicuous [rf. Thomas, 1 888, p. 127, profile figure). The upper lateral 

 teeth of Petaurus are like those of the smaller forms, and the same is true of Dactylopsila, 

 except tliat here the edge of the posterior tooth {i. 3) is much broader, and its anterior 

 portion slightly more inwardly rotated than in Petaurus. In both genera the lower 

 teetli are elongated and recurved, as in the smaller species, and this character is again 

 more pronounced in Dactylopsila tlian in Fetawus. 



Phakmger and Trichosurus show a more specialized modification of the incisors, but 

 it is of a different type from that seen in the preceding forms. The procumbent develop- 

 ment of the median upper teeth, which is so pronounced in the next lov>'er member of 

 the series, Dactylopsila, is here scarcely in evidence. In Dhalanger these teeth are 

 more rounded in section, and their length is reduced, so that their tips project only 

 slightly beyond those of the lateral teeth. In Trichosurus the length is still further 

 reduced, and, except for their greater curvature, they begin to show the characters of 

 the lateral teeth. In Phalanger ursinus and P. melanot is the upper lateral teeth are 

 much like those of Petaurus and Dactylopsila, the second serving chiefly as a stop for 

 the lower ones, while the third, which are again spatulate at their tips, have their 

 anterior portions rotated inwards. The remaining species of Phalanger show a 

 specialized condition, the third upper teeth being reduced through the encroachment of 

 the canines. In Trichosurus the upper Literal teeth are like those of P. ursinus 

 and P. inelanotis. In both genera the lower teeth are flattened, lanceolate, and only 

 slightly curved. In neither case do they show any tendency to continue the progressive 

 elongation and piercing developments characteristic of the lower members, especially 

 Dactylopsila. 



Tlie incisor modifications just described furnish the first instance of the conflict of 

 developments above referred to as characterizing the omnivorous evolution of the 

 Phalangerinae. In the minute forms Acrobates, Distcechurus, and Dromicia the 

 insectivorous purpose of the diprotodont modification is fully realized, the procumbent 

 character of the median upper teeth, and the elongated, recurved, and sharply-pointed 

 character of the lower ones, combining to make these structures of obvious value in the 

 prehension of food. Proceeding from these more or less initial forms we meet with a 

 series of genera showing a gradual increase in the size of the body, Avhich we might 

 reasonably suppose, both on general principles and from the analogous case of the 

 Dasyurinse, to demand the abandonment of insectivorous habit and a gradual reduction 

 of insectivorous characters. What we do find in the present case, however, is a tendency 

 to continue the insectivorous developments. This is not surprising in the case of 

 Petaurus, especially the smaller species P. breviceps and P. sciureiis, but it certainly is 

 so in the case of Dactylopsila, both on account of its larger size and its actually 

 insectivorous habits. Passing to the still larger forms Phalanger and Trichosurus we 

 find the conditions abruptly changed, the insectivorous characters being rej)laced by 



