126 FIELD MUSEUM OF NATURAL HISTORY ZOSLOGY, VOL. XIV. 



functional importance of the metacone in polyprotodonts. Gregory 

 (1916, p. 248) has suggested even that the metacone in these forms 

 represents the apex of the original two-rooted tooth. It is to be noted, 

 however, that with the reduction of the metacone spur and the growth 

 of the hypocone the relative importance of the metacone decreases and 

 in some cases (e. g. Dactylopsila) the postero-external cusp, presumably 

 the metacone element, is exceeded by the hypocone which was non- 

 existent when the metacone dominated the tritubercular crown. In 

 Phascologale, Antechinomys and other generalized dasyurids, the stylar 

 element (style c) is very well-developed and only slightly exceeded by the 

 metacone. According to Sinclair, style c is well-developed also in 

 Microbioiherium tehuelchum of the Patagonian Miocene. To totally 

 eliminate this style and change the selenoid form of the metacone to 

 bunodont, meanwhile transposing the relations of the two to the root, 

 would be a much more extensive process than fusion of style and meta- 

 cone or even more than increased growth of the style and elimination 

 of the metacone. Moreover, it would be contrary to general evidence on 

 cusp migration, most of which is negative (Gidley, 1906; Tims, 1903). 



LOWER PREMOLARS. 



Description. The single-rooted, unicuspid, anterior lower premolar 

 lies in series with the canine and the lateral incisors and is essentially like 

 them in size, form, and implantation. It is separated from the middle 

 premolar by a slight space. In the abnormal specimen having five 

 unicuspids, this space is unaffected, indicating that the extra tooth is 

 an anterior rather than a posterior one. 



The middle premolar is two-rooted and tricuspid but it is in a 

 rather advanced stage of reduction toward the unicuspid condition. 

 The principal or middle cusp has taken an anterior position directly 

 above the anterior root leaving a long sweeping curve between it and 

 the posterior cusp. The anterior cusp is practically eliminated, although 

 its position is still indicated by the angle between the root and the front 

 of the main cone. A further reduction involving the loss of the small 

 posterior cusp and root would produce a tooth like the unicuspid 

 anterior premolar. 



The posterior lower premolar, like the corresponding upper tooth, 

 is functionally associated with the molars rather than with the an- 

 terior premolars. A slight space separates it in front from the middle 

 premolar but posteriorly it is closely engaged with the front of the 

 first molar. It rises considerably above the anterior teeth and even 

 exceeds slightly the level of the molars. It has one large upright cusp 

 rising above its anterior root and a short depressed heel which is over- 



