1 50 Bulletin American Museum of Natural History. [Vol. XIV, 



are from the United States, 187 skins and 34 skulls from Mexico 

 and Guatemala, 10 from Costa Rica, 26 from Colombia, 3 from 

 Venezuela, and 12 from Trinidad. While this material far ex- 

 ceeds that previously available for the study of the group, it is 

 very inadequate for more than a preliminary survey of the field. 

 Besides the lack of material, the study of the group is rendered 

 exceptionally difficult in consequence of the wide range of in- 

 dividual variation these Opossums present, and also by the 

 peculiar character of the pelage, and the lack of any well defined 

 pattern of coloration. 



Beginning with the Opossums of the United States and pro- 

 ceeding thence southward to northern South America, several 

 well marked features of variation are prominent. While at the 

 North the animal is at about its average size, judged especially 

 by the skull, there is found in proceeding from the North south- 

 ward a very marked increase in the length of the tail, which in 

 the northeastern States is less than 70$ of the length of the head 

 and body, while in tropical latitudes its length often equals 

 the length of the head and body, sometimes exceeds it, and gen- 

 erally falls but little short of it. The" ears are small in the 

 northern animal in comparison with its southern representatives, 

 in tropical examples the ear having far greater superficial area 

 than in extreme northern specimens. 



The coloration also varies markedly, not only as regards the 

 pelage but in respect to the ears and tail. In the United States 

 (the D. virginiana group) the dark basal portion of the tail gener- 

 ally extends little if any beyond the portion covered by the long 

 hair at its base. In the Mexican and South American forms the 

 dark color usually occupies the basal third, often one half, and 

 sometimes two thirds of its length. In the northern animal 

 the ears are black, tipped more or less broadly with flesh color, 

 the amount decreasing southward, even to some extent within the 

 United States; while in tropical latitudes the ears are wholly 

 black in all the forms of the Didelphis group, except in D. aurita 

 of southeastern Brazil, in which they are again tipped with flesh 

 color, and in D. pernigra of Peru, in which they are wholly white. 



The color of the feet also varies geographically, at the north 

 the toes on all the feet being usually white apically, but the white 

 extends further up the toes on the fore feet than on the hind feet. 



