1922] Stock-Furlong : A Marsupial from the John Day Region 315 



Perathcrium. Opposite the notch between paracone and metacone and 

 on the external cingulum is situated a small elevation which is flanked 

 behind by a larger cuspule, the latter being comparable perhaps to 

 stylar cusp c of Bensley. 4 



In M- the metacone is also larger than the paracone, the difference 

 is similar to that seen in the corresponding tooth of Marmosa and is 

 not so great as that occurring in M- of Didelphys. This tooth, in 

 contrast to the other molar teeth, possesses the greatest number of 

 distinct elevations on the external cingulum. Two of the elevations 

 are situated at the antero-external angle of the tooth and are perhaps 

 comparable to the pair situated in this region in M 1 . Opposite the 

 notch between paracone and metacone may be seen two minor eleva- 

 tions with a somewhat more pronounced stylar cusp situated immedi- 

 ately behind. The latter is probably comparable to stylar cusp c of 

 Bensley. The ledge which extends along the anterior base of the 

 paracone from the most anterior stylar cusp is stronger than in M-. 



M- is less robust than M-. In M- the paracone closely approaches 

 the metacone in size. In the Logan Butte specimen a single stylar 

 cusp appears to be present on the external cingulum at the anterior 

 end of the tooth from which a well defined ledge extends along the 

 anterior base of the paracone. A rather prominent stylar cusp is 

 situated on the external cingulum opposite the notch between paracone 

 and metacone. 



A fairly prominent cuspule is developed on the cingulum at the 

 antero-external angle of M- and from this a ledge extends along the 

 anterior base of the principal V-shaped cusp of the crown. Opposite 

 the middle of the principal cusp occurs a small elevation on the cin- 

 gulum. Examination of M- in three skulls of Marmosa failed to reveal 

 the presence of an elevation in this region of the cingulum. In several 

 skulls of Didelphys, however, two distinct disputes are present on the 

 external cingulum. The posterior ridge or arm of the principal V- 

 shaped cusp in M- of Perathcrium merriami, as in that of P. fugax, 

 joins the cuspule or style located at the postero-external angle of the 

 tooth. The postero-external style or cuspule is not prominent in M- 

 of the North American Oligocene Perathcrium. In M- of the Recent 

 Marmosa the postero-external style is well defined. The A r -shaped 

 cusp is not so prominent in M 1 of Marmosa as in that of P. merriami, 

 but the inner cusp is much better developed than in M- of the latter 

 form or of P. fugax. 



i Ibid., p. 152, fig. 1. 



