MAESUPIALIA. 789 



There are five species of Peratherium known to me, two of Domnina, 

 and possibly of Mesodectes, and one of Menotheriwn. 



FlO. 32. — Skull of opossum {Didelphya rriginiana), uatural size, posterior view, parts of the right 

 mastoid and squamosal boues removed. M, mastoid foramen; SBS, subsquamosal ; PG, postglenoid; 

 PZ, postzygomatlc foramen. 



PERATHERIUM Aymard. 



Merpetotherium Cope. Paleontological Bulletin, No. 16, August, 1873, p. 1. Synopsis of New Vertebrata 

 from the Tertiary of Colorado (Misc. Pub. U. . Geol. Surv. Terrs.), October, 1873, p. 4. Annual 

 Report U. S. Geol. Surv. Terrs., F. V. Hayden in charge, 1873 (1674), p. 465. 



113 4 

 Dental formula: I. -; C. ^-j Pm. -; M. -. The superior canines are well 

 4 13 4 



developed. The premolars are compressed, with one apex, no inner lobes, 

 and two roots. The superior molars, excepting the last, present two median 

 Vs, which would be termed external but for the fact that the external basal 

 cingulum is so developed as to constitute an external crest. A single internal 

 lobe, which is less elevated than the median. The last molar has but one 

 median and one internal V. The superior incisors are unknown. The in- 

 ferior incisors are subequal and closely placed; the first three ai-e parallel, 

 Inferior canine well developed, recurved. Inferior premolars compressed, 

 simple unicuspid. Molars consisting of two Vs, with apices external. The 

 inner extremity of each branch of the anterior V is a pointed cusp, behind 

 which stands, on the inner edge of the crown, a third cusp opposite to the 

 middle of the posterior V. The last inferior molar is little shorter than the 

 others, but the second V is nan-owed so as to be only a heel. The molars 

 do not materially enlarge anteriorly, and the posterior mental foramen is 

 not posterior to the first true molar. 



