1902.] Allen, Opossums of the Genus Didelphis. 255 



Brazil. It is thus an animal of the Cordilleras, the mountainous 

 portion of southern Brazil, and the more southern pampas. 

 This, at least, is the evidence afforded by the present material 

 and the literature of the subject. The two groups thus over- 

 lap each other geographically only in southern Brazil, northern 

 Argentina, and in parts of Bolivia, and again at the northward 

 over limited portions of Colombia and Venezuela. 



In respect to the distribution of these two groups in Ecuador 

 and Peru, it is of interest to note that the late Mr. P. O. 

 Simons, on his collecting trip for the British Museum during 

 the years 1898-1901, from the coast region of Ecuador south- 

 ward and eastward to southeastern Peru and the adjoining 

 part of Bolivia, sent only specimens of the marsupialis group 

 from the low coast region west of the Andes, from the De- 

 partment of Piura in Peru northward to Guayaquil, and 

 only specimens of the paraguayensis group from the interior 

 of Ecuador, Peru, and Bolivia. 



The marsupialis group is not very sharply separable from 

 the large opossums of Mexico and the United States. Neither 

 intergradation between the North American and South Ameri- 

 can forms, nor the reverse, has as yet been established, owing 

 to the lack of material from large portions of Central America. 

 It is evident, however, that the relationship is close among 

 all these forms, in comparison with their sharp differentiation 

 from all the members of the paraguayensis group. 



The extraordinary amount of variation in individuals of 

 the same subspecies, from even the same locality, in respect 

 to size and coloration, in the relative length of the tail and 

 in cranial characters, combined with dimorphic conditions in 

 coloration, renders the discrimination of local forms a very 

 difficult matter, and at best, with the limited material as yet 

 available for examination, exceedingly unsatisfactory. Be- 

 ginning at the northward, we have, first, the large virginiana 

 type, white-headed, short-tailed, and light-colored, with the 

 margin of the ears and the tips of the toes and much the 

 greater part of the tail white, and never, so far as known, 

 running into a melanistic phase; in its southern subspecies, 

 pigra, the underfur is more extensively tipped with black, 



