1922 .1 Stock-Furlong: A Marsupial from the John Day Region 313 



Description. — Specimen 24240 is smaller than skulls of Recent 

 species of the South American polyprotodont genus Marmoset. The 

 palate (fig. 1) in the John Day specimen has large fenestrae between 

 the molar series, but the largeness of the fenestrae is in part due to 

 a breaking away of the palatal margins. The tympanic process, which 

 is bullate in Marmosa and formed by the alisphenoid, is as much 

 developed in Peratherium merriami as in the former genus. In side 

 view of skull (fig. 2), the exit of the infra-orbital canal is seen to be 

 situated farther in front of the anterior border of the orbit than in 

 Marmosa. In P. merriami the distance measures 3.3 mm., while in 

 four skulls belonging to three species of Marmosa the measurement 

 varies from 1.8 mm. to 2.8 mm. The exit is located above the anterior 

 end of the first molar in P. merriami; in Marmosa it is often seen above 

 the middle of the last premolar. In Peratheri{um fugax the facial exit 

 of the infra-orbital canal is also situated at a greater distance from 

 the orbit than in Marmoset. 



, ; '7 



Figs. 1 and 2. Peratherium merriami, n. sp. Skull, no. 24240, Mus. Palae. 

 Coll., X 2. Fig. 1, ventral view; fig. 2, lateral view. John Day beds, Logan 

 Butte, Oregon. 



In Peratherium merriami and in P. fugax the anterior border of 

 the orbit lies above the posterior end of M-; in Marmosa the border 

 may be situated above the middle or the front end of M-. The orbit, 

 therefore, does not extend so far forward in the Oligocene species as 

 in the Recent Marmosa. The anterior border of the orbit in the Recent 

 Dielclphys reaches forward to a point situated above the posterior end 

 of M 1 while the facial exit of the infra-orbital canal is located above 

 the posterior end of P-. The zygomatic process of the squamosal in 

 no. 24240 is relatively shorter than in skulls of Recent Marmosa. 



In the lower jaw the inferior border below the masseteric fossa is 

 not deflected decidedly from that beneath the tooth-row. The angle 

 is deflected inward as in modern didelphids. A mental foramen is 



