SINCLAIR: MARSUPIALIA OF THE SANTA CRUZ BEDS. 



409 



FIG. 6. 



Plata Museum (text-fig. 6) shows that this tooth supports a prominent 

 central cusp, but the minuter details can not be ascertained from the 

 photograph. The first two molars are of the same size, the third is a 

 little narrower transversely and the fourth greatly 

 reduced. All, except the fourth, are triangular in 

 outline and tricuspidate, with conic cusps, a nar- 

 row external cingulum and reduced metacone 

 spur. An antero-external style, designated by 

 Bensley, style ab (Bensley, 1903, pp. 89, 183 et 

 seq.}, is present on all the molars as a distinct 

 tubercle. Style c is well developed on the second 

 and third molars of Microbiotherium tehuelchum, 

 but is only slightly differentiated from the cingu- 

 lum in M. tortor. On the anterior molars the 

 metacone is decidedly larger than the paracone. 

 The metacone spur is greatly reduced compared . Mi ^ tf > S P-- P alatal 



... r * view of the skull, x I. The 



With Its Condition in Dldelpliys or DasyuntS, representation of the molar 



Owing to the decreased width of the Cingulum. patterns is slightly diagram- 



M- has the metacone vestigial. The cusps rep- matic - Drawn from an en- 

 resented on this tooth are the protocone, para- larged P hot g ra P h of a s P ed - 



. 7 , . ... _ . men in the La Plata Museum. 



cone, style ab and the vestigial metacone. The 



protocone is supported on a separate root, as in Didelphys. On the three 

 anterior molars the protocone encloses a basin-shaped depression, on the 

 margins of which two minute cuspules are developed. In shape and 

 pattern, these teeth resemble closely the molars of some of the subspe- 

 cies of Caluromys. All the upper molars are triple-rooted. 



The lower incisors are spatulate in shape, resembling the incisors of 

 Dasyttnts rather than Didelphys. Unlike these genera, the root of the 

 second tooth in the series is not displaced posteriorly with reference to 

 the roots of the first and third. The first and second incisors only are 

 preserved in the mandible associated with skull No. 15,698. The canine 

 is either short, blunt, and directed anteriorly (M. tortor, PI. LXII, fig. 2), 

 or disproportionately long and pointed (M. teJmelcluiin, PI. LXII, fig. 4). 

 The three lower premolars are double-rooted and are either closely ap- 

 proximated (M. tortor, M. patagonicitm], or spaced (M. tehtielchnm}. The 

 anterior premolar is a small, simple-crowned tooth, situated close to the 

 canine. The median and posterior premolars are provided with prom- 



