126 DE. B. A. BENSLEY ON THE EVOLUTION 



apart from the diprotodont modification, in their dental characters, make an interesting 

 approximation to the insectivorous Dasyurinoe, while the most specialized forms, such 

 as Fhdkoiger and Trichosurus, iu their large size and. incipient lierbivorous modifi- 

 cations, present an opposite extreme, bordering on the conditions in the smaller of the 

 Macropodidse. 



It may be observed, however, that the sequence of dental modifications in the present 

 division does not indicate an evolution as completely homogeneous as that of the 

 Dasyurinae. Each genus will be seen to present one or more specialized modifications 

 which, while not disturbing the general sequence, prevent it from being wholly 

 prototypal to a succeeding one. The cause of this condition will be better discussed, 

 after a review of the dentition of the group, but it may be mentioned at this point that 

 the omnivorous-herbivorous evolution, preceded as it is by the diprotodont modification, 

 appears to represent a critical stage in the general dental progression. There appears 

 to liave been a double conHict between the continuation of the reduction of the 

 posterior premolars and their elaboration as sectorials, and between the use of the 

 incisors as piercing and cutting organs and the transference of their functions to 

 the posterior premolars. The molars will be seen to show an evolution as gradual 

 as that of the Dasyurinae and Peraraelinae, the only ijuportant exception being that in 

 Acrohates, Dlstoechurus, and two species of Bromicia there is a reduction of the formula 

 by one tooth above and below. 



Sequence of Blolar Patterns. — Apart from certain characters of specialization, to which 

 reference will be made below, the molars of Dlstoechurus pennatus, the form selected for 

 illustration, may be taken as representing tlie starting-point for the evolution of the 

 bunodont section, as far as the stages of this are indicated in recent forms. 



The upper molars of Dlstoechurus decrease in size from before backwards, the first 

 tooth being more than three times as large as the tiiird. This character and also 

 that represented by the reduced condition of the molar formula are secondary and 

 not prototypal. Each of the upper teeth (PI. 5. tig. 13) is roughly triangular in 

 shape, and shows only four cusps, representing the protocone, paracone, metacone, and 

 hypocone {cf. p. 89, text-flg. 1). The protocone and hypocoue are placed below the level 

 of the remaining cusps, as in the Peramelinae and, in the case of the protocone, the 

 Dasyurinte and Didelphyida). Together they appear to form an internal ledge attached 

 to the bases of the outer cusps. The hypocone is not fully developed, and this accounts 

 for the somewhat triangular contour of the crown. Both the paracone and metacone 

 are well developed, and present a very different appearance from that seen in the 

 Dasyurinae and Peramelida^, there beiag no disproportion in their relative sizes, and no 

 tendency towards any excavation of their external faces, so that, apart from a fine 

 trenchant line extending over their tips, they are wholly bunoid. External styles are 

 entirely unrepresented. There are also no traces of a metacone-spur. 



The lower molars of Dlstoechurus do not show the disproportion in size characteristic 

 of the upper teeth. The first is only slightly, if at all, larger than the second, and the 

 latter is slightly larger than the third. The second (PI. 6. fig. 18) and third arc oblong 



