OF THE AUSTRALIAN MARSUPIALTA. 153 



Posterior Premolars. — As in the Bettongiinae and Potoroinae, these teeth furnish the 

 most decisive evidence concerning the sequence of the different species. Leaving out of 

 consideration for the present the genvis Setomjx, the most primitive conditions are found 

 in tlie two species Dendrolagiis Lumholfzi and D. Bennettlaiius. Tn both forms the upper 

 teeth are scarcely as long as the first and second molars. Their length is about equal to 

 that of the upper incisor rows. They are comparatively l)road, but the cutting-edge is 

 thin and emarginate. The edge ends anteriorly in a pyramidal column, probably repre- 

 senting the protocone. There is a prominent internal ledge, ending posteriorly in a well- 

 developed cusp like that seen in Caloprijmmis. There is also a postero-external cusp. 

 In the Papuan species, B. Boriamis, we find a more advanced stage of sectorial evolution. 

 The teeth are of about the same size as in the preceding forms, and show the same 

 anterior pyramidal cusp, internal ledge, and postero-internal cusp. The postero-external 

 cusj) is, however, absent ; and the internal ledge is present in the form of two rounded 

 protuberances. The emarginate portion of the edge is furthermore thickened, and now 

 bears two distinct vertical ridges with corresponding grooves. In the remaining Papuan 

 form oi DendroUtgns, D. hmsins, we find a transitional stage between D. Doriaims and 

 D. Lmnholtzi, the cutting-edge of the premolars being thin, the internal ledge not in the 

 form of protuberances, the ridges scarcely distinguishable, and the postero-external cusp 

 absent. 



Passing to Dorcopsis, we find a more advanced stage of the same evolution. In 

 D. Macleayi the teeth are slightly longer than the first and second molars, and decidedly 

 longer than the upper incisor rows. The edge is uniformly thickened and not 

 emarginate, so that the differentiation of the anterior pyramidal cusp is not so marked. 

 The sides of the teetli are marked by four prominent ridges, with indications of a fifth, 

 the most anterior ridge corresponding to that formed by the anterior pyramidal cusp in 

 Dendrolagus. As before, there is a postero-internal cusp, but no postero-external one. 

 There is probably an internal ledge : this part of the teeth was found to be worn in the 

 single specimen examined. In D. luctuosa the teeth have the same characters as in the 

 preceding species, except that they are still more elongat'^l, being now equal in length to 

 the first and second molars together with the tip of the anterior crest of tlic third. There 

 are again four ridges, with indications of a fiftli. The internal ledge is prominent and 

 tuberculate, as in JJ. Doriamis. In I). Mueller I we find the last stage of the evolution. 

 The teeth are exactly similar to those of Z>. luctuosa, except that they are again more 

 elongated, their length being now equal to that of tlic first and second molars together 

 Avith the whole anterior lobe of the third. The character of the internal led^e is 

 dovibtful. 



In Setonyx the outer premolars show the same stout proportions as those of Dendro- 

 lagus. Their length is about equal to that of the first and second molars combined. In 

 this character they approximate to Dendrolagus LumhoUzi and D. Bennettianus. There 

 are no traces of a postero-external cusp found in the latter forms. The cutting-edge is 

 not emarginate ; it bears three stout ridges, with indications of a fourth. Tliere is a 

 well-developed postero-internal cusp and an internal ledge which does not appear to be 

 tuberculate. 



