152 DE. B. A. BEJS'SLEY ON THE EVOLUTION 



premolars it is, however, more primitive than Bendrolagus Lumholtzi, while in the 

 extent of reduction of its canines it is more advanced than the final members of 

 Dorcopsis. There is no evidence that the dental characters of this form are the result 

 of arboreal habit. Thomas (1888) estimates its systematic position to be with the members 

 of the small Wallaby section of the genus Macropus, and it probably represents an 

 aberrant form of this group which has assumed feeding-habits similar to those of the 

 arboreal forms. 



Molars. — The characters of these teeth are almost constant throughout the series. In 

 all forms the tooth-rows are either straight or but slightly bowed outwards in the middle. 

 The upper teeth increase gently in size from before backwards from the first to the third, 

 the fourth being slightly smaller on account of the meagre development of their posterior 

 lobes. The lower teeth increase in size from the first to the fourth. These proportions 

 are more primitive than those seen in the true Kangaroos, but represent an advance on 

 those of the Potoroinae and Bettongiinse. All the teetli are functional at the same 

 time, the first teeth being only partially worn when the fourth are fully in place. Their 

 tips accordingly project to the same extent, and thus present a very diff'erent appearance 

 from that seen in the A'\'allabies and Kangaroos, where the functional rotation of the 

 teeth jiroduces an arcuate arrangement of the tips. Both in the upper and lower teeth 

 {cf. PI. 5. fig. 20; PL 6. fig. 26) the originally anterior and posterior pairs of cusps are 

 completely connected by transverse crests. In this character and in the relative 

 shortness of the crests they represent a transitional stage between the cuspidate 

 condition of the Potoroinae and Bettongiinse and the bypsilophodout condition of 

 the Wallabies and Kangaroos. Longitudinal crests formed by the modification of the 

 original longitudinal bands arc indicated, and only to a moderate extent, in the median 

 valleys of the upper molars. 



Incisors. — Although essentially macropodine rather than phalangerine in their general 

 characters, these teeth are noticeable for their comparative pi'imitiveness. The median 

 upper incisors are rounded in section, but slightly flattened at their tips. They tend to 

 project slightly beyond the lateral teeth. The latter are comparatively small, and appear 

 to serve as much for grasping as for cutting organs. Their crown-surfaces are basin-like, 

 the rimis of the basins being, however, incomplete externally. This modification recalls 

 that seen in the incisors of the Equidae, the pseudo-invagination having, however, taken 

 j^lace internally in the latter. The lower incisors are distinctly lanceolate and sharp- 

 edged, as in the members of the hypsodont series. In Setonyx the incisors are more as 

 in the hypsodont series, both median and lateral upper teeth being distinctly flattened, 

 the low^er lanceolate. 



Canines. — These teeth are present in a reduced condition in all the species of 

 Bendrolagus and Dorcopsis. They are in general more primitive as regards the extent 

 of reduction than in the hypsodont series ; in the latter they are, as a rule, either wholly 

 vestigial or absent. In Selonyx, notwithstanding its small size and general primitiveness, 

 the canines have wholly disappeared. 



