or THE AUSTRALIAN MAESUPIALIA. 133 



P. melanolis, P. orientalls, and P. maculaliis we find that the original tip (protocone) 

 is anterior and more or less external in position, and that the posterior slope is com- 

 pressed and slightly trenchant. The cutting-blade thus formed bears one or two short 

 superficial grooves ending in notches at the edge. In P. melcmotis, where only one 

 groove is present, the teeth resemble those of Dromicia nana and 7J. lepida. In all 

 cases the posterior portion of the trenchant edge tends to be elevated, Init tliis tendency 

 is much more apparent in certain forms {P. orientalls) than in others (P. hreviceps, 

 P. celebensis). In P. lullulce, P. lencippus, and P. HothschilcU the trenchant character 

 of the edge is much more pronounced, tlie posterior portion of the edge being still more 

 elevated and bearing three grooves. 



An interesting feature of the development of the trenchant edge is its axial rotation. 

 In P. melanolis, P. ttrsinus, and P. maculatus we find a practically unrotated condition, 

 so that a line drawn through the cutting-edge, if it passes through the middle line of the 

 skull at all, will pass posteriorly in the region of the condyles. In the case of P. orientalls, 

 P. lullulce, and P. leuclppus it will pass through the middle line forwards in the region 

 of the palatal vacuities. Increase in relative size seems also to be a part of the sectorial 

 elaboration. In P. celebensls and P. hreviceps the teeth are small, while in such extreme 

 forms as P. leucip)pits and P. Rothschlldl they are considerably enlarged, now exceeding 

 the first molars. 



It is interesting to note at this point that in the Macropodidre the sectorial evolution 

 has proceeded along two lines, one of which, represented by Hi/psiprynmodon and 

 Bettongia, appears to present in an extreme degree the grooved and rotated condition 

 found in Fhalanger, while tbe other seems to represent a modification of the simpler, 

 practically ungrooved and unrotated type found in Dromicia and Petavrus. 



In Trichosurus the upper anterior premolars are single-rooted, as in the two preceding 

 genera, Fhalanger and Dachjlopsila ; they are on the whole, however, more reduced. 

 The median teeth have now disappeared. The posterior premolars above and below 

 show much the same conditions of sectorial elaboration as are seen in the more 

 specialized forms of Fhalanger. Their trenchant edges are elevated, as in P. leucippm, 

 and are rotated to about the same extent as in the latter form or P. orientalis. They 

 show indications of three notches. A significant feature is that the sectorial teetii 

 show, as a rule, more decided indications of wear in Triehosurus than in Fhalanger, 

 this character supplementing the evidence of the molars, canines, and incisors as to tlic 

 greater herbivority and more advanced position of the former genus. 



Apart from special phylogcnetic considerations, the modifications of the premolars in 

 Fhalanger and Trichosurus possess a general significance. While these genera may be 

 shown by their lack of development of the median premolars to be not very nearly 

 related to the ancestors of the Macropodidtc, they show the incipient phases of a sectorial 

 development of the premolars, which is continued and perfected in the latter family. The 

 fact that the formation of sectorials may have taken place not only within the limits of 

 the family but within the limits of two genera, indicates the futility of such general 

 comparisons as have been made in suggesting affinities between the specialized sectorials 

 of the Macropodidse and those of certain of the South-American Miocene forms, or of tlie 



Plagiaulacidse. 



19* 



