HISTORY OF THE MARSUPIAilA 631 



The division of the existing opossums into genera has 

 caused much difference of opinion and practice among nat- 

 uraUsts ; there are five groups, which by some are regarded as 

 genera, and by others as subgenera, all modem members of the 

 family being very much aUke. The species Didelphis marsvr- 

 pialis, which is common in the eastern United States and ex- 

 tends through temperate North America, Central America and 

 tropical South America, has a complete pouch and is chiefly 

 arboreal and insectivorous in habit. In the woolly opossums 

 (Calwomys) there is no pouch, and the young, when sufficiently 

 advanced, are carried on the mother's back, winding their 

 tails aroimd hers. In both of these genera the tail is long, 

 naked and prehensile, but in the tiny species of Peramys the 

 tail is short and hairy. Another Neotropical genus, Chironectes, 

 the Yapock or Water Opossum, is the only existing instance of 

 an aquatic marsupial. It has Ught grey fur, striped with brown, 

 and webbed hind feet ; Mving chiefly in the water, it feeds upon 

 crayfish, water-insects and small fish. 



The derivation of the modem North American opossums 

 is a matter of great vmcertainty. The present distribution 

 of the family, with by far the greater number of its species 

 confined to the Neotropical region, is certainly suggestive of 

 a South American origin, but such considerations are very 

 untrustworthy guides in tracing the history of animal groups. 

 No opossimi has been found in any North American formation 

 between the Pleistocene and the lower Ohgocene, though in 

 the case of such small animals, negative evidence must be 

 accepted with caution. In the White River Ohgocene many 

 minute opossxmis have been foimd and referred to the European 

 genus ^Peratherium, though it so closely resembles the modem 

 Didelphis that many systematists do not make the distinction. 

 In the Eocene, Paleocene and upper Cretaceous, opossums 

 were represented doubtfully; the material is too incomplete 

 for assured determination ; in Europe they existed in the Oh- 

 gocene and upper Eocene. In South America the family 



