124 FIELD MUSEUM or NATURAL HISTORY ZOOLOGY, VOL. XIV. 



and elimination of styles. The high bladelike cusps in the molar of 

 Ccenolestes suggest an evolution along lines similar to those which have 

 produced the carnivorous dentitions. In the Dasyuridae, the carnivorous 

 evolution obviously has proceeded by lateral compression and fusion of 

 styles and cusps. In this process it is difficult in all cases to demonstrate 

 to what extent one element dominates the other, but the styles are 

 quite as consistent as the so-called cusps of the trigon. In Sarcophilus, 

 the style dominates the paracone in the first and second molars and the 

 two are evenly divided in the third. Metacone and style, on the other 

 hand, are evenly divided in the first molar, while the metacone dominates 

 in the third. The character of the outer aspect of the teeth in all cases 

 seems determined by that of the styles. In Thylacmus, stylar elements 

 persist in reduced condition anteriorly and posteriorly but in the inter- 

 vening region have completely fused with the metacone, doubtless some- 

 what after the manner indicated in Sarcophilus. Even after these 

 extensive changes, the resulting cusp, which is derived principally from 

 the metacone, is situated in the same relation to the roots as the meta- 

 cone of the tritubercular molar of the didelphids and dasyurids. In 

 the carnivorous evolution, therefore, although the tendency to con- 

 solidation of cusps is very great, the styles have been reduced only 

 when their corresponding cusps were reduced and in cases of fusion they 

 have given their character to a large part of the combined product. 



In the formation of the molar of Ccenolestes with its decidedly convex 

 labial surfaces it seems probable that fusion of styles has played quite 

 as large a part as in carnivorous forms. In the bunoid Phalangerinae it 

 is possible that fusion and reduction may have been combined in some 

 cases. As compared with the upper molars of other living marsupials, 

 those of Ccenolestes are unique in the possession of a marked undiffer- 

 entiated external cingulum. It forms a continuous faintly crenulate 

 ledge along the bases of all the molars. A similar structure is found in 

 various degrees of perfection in the extinct casnolestids but nothing 

 exactly comparable occurs in modern forms. It is not necessarily 

 evidence of reduction of styles and may easily have been developed 

 independently. In fact an exactly analogous structure is found at the 

 inner base of the protocone of the first upper molar and cingula of the 

 same character appear on the lower molars to which no suspicion of 

 reduced styles attaches. The irregular swelling at the bases of the 

 upper molars in primitive Phalangeridae, as Dromicia and Petaurus, is 

 regarded by Bensley as a vestigial external cingulum which "appears to 

 be equivalent to that bearing the external styles in polyprotodont forms." 

 In a certain sense this is apparent enough, but I believe that at most 

 only the minor stylar elements are represented and that the principal 



