SINCLAIR : MARSUPIALIA OF THE SANTA CRUZ BEDS. 
409 
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. tor tor. On the anterior molars the 
metacone is decidedly larger than the paracone. 
The metacone spur is greatly reduced compared . f „ 
1 ° J 1 view of the skull, x -f. the 
with Its condition in Didelphys or Dasyiinis, representation of the molar 
owing to the decreased width of the cingulum, patterns is slightly diagram- 
M A 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 eci_ 
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 
Dasyurus 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. tehuelchum , PI. LXII, fig. 4). 
The three lower premolars are double-rooted and are either closely ap- 
proximated (M. tortor , M. patagonicmn ), or spaced (M. tehuelchum). The 
anterior premolar is a small, simple-crowned tooth, situated close to the 
canine. 1 he median and posterior premolars are provided with prom- 
