MAY, 1921. AMERICAN MARSUPIAL, C^ENOLESTES OSGOOD. 125 



ones (styles 6 and c} are no more reduced than they are in the anterior 

 molars of Sarcophilus in which there is a similar slight basal ridge com- 

 bined with large obvious styles. 



Evidence of combined fusion and reduction is seen in the first upper 

 molar of Dactylopsila. In this the quadrate form is complete so far as the 

 principal cusps are concerned, but there remains on the antero-external 

 border of the tooth a single well-marked style, which is well distinguished 

 from the paracone. Apparently the other styles have fused, but this 

 one has changed only little and that by reduction. It is to be noted also 

 that this style retains a primitive relation to the root. Most interesting 

 is the fact that the antero-external surface of the paracone, that is, the 

 part opposite the style, is concave or semi-selenoid while its postero- 

 external surface is convex like that of the succeeding molars. In other 

 words, it seems that where fusion takes place, the outer surface of the 

 style is added to the cusp making it convex and where fusion does not 

 take place, the outer surface remains concave as at first. 



To produce a bunoid molar from a tritubercular one by this method 

 requires less change than by reduction of styles. In the primitive 

 Dasyurinae and in Marmosa and Peramys the styles are very well- 

 developed and their convex external surfaces and sublanceolate form 

 closely parallel the conditions in Ccenolestes and some of the primitive 

 Phalangerinae. Moreover, the paracone in these forms is obviously 

 derived from the styles, in most cases being subordinate to them and in 

 none reaching a stage of development which overshadows them. In the 

 first upper molar of Phascologale cristicauda the paracone is not yet 

 differentiated and in the corresponding tooth in Dasyurus viverrinus 

 nearly the same condition obtains. This tooth, on account of its anterior 

 position, is probably more primitive than those posterior to it. In these 

 forms, its antero-external cusp has all the characteristics of a style 

 (style 6). A very slightly bifid tip indicates the beginning of the separa- 

 tion of the paracone and its complete differentiation is seen progressively 

 emphasized in the succeeding third and fourth molars. This first upper 

 molar in the dasyurids is at least superficially similar to the deciduous 

 molar of the didelphids in which, as suggested by Bensley, the antero- 

 external cusp appears to be the result of fusion of style and paracone 

 and may be retrogressive. However this may be, the tendency to 

 persistence of the styles is evident and the conclusion seems justified 

 that the same elements are still represented in the quadrate bunoid 

 molars of diprotodonts. If this be true it is possible that the small 

 metaconule in the molar of C&nolestes is not a new element but a per- 

 sistent unfused vestige of the metacone of the ancestral tritubercular 

 tooth. This at first seems improbable on account of the large size and 



