DIDELPHYID& 135 



fourth or Micoureine group the numerous species are all smaller 

 than in the preceding groups, and have short and close hair, and 

 no dark streak down the face. The best known species is the 

 Murine Opossum (D. murina), little larger than a House-Mouse, 

 and of a bright red colour, which is found as far north as central 

 Mexico, and extends thence right down to the south of Brazil. The 

 last or Peramyne group contains several extremely shrew -like 

 species, of very small size, with short, hairy, and usually non-pre- 

 hensile tails, not half the length of the trunk, and with wholly 

 unridged skulls. The most striking member of the group is the 

 Three-striped Opossum (D. americana), from Brazil, which is of a 

 reddish-gray colour, with three clearly-defined deep-black bands 



FIG. 38. The Woolly Opossum (Didelphys lanigera). 



down its back, very much as in some of the striped mice of 

 Africa. 



The numerous fossil species of Opossum found in the Upper 

 Eocene and Lower Miocene of Europe are of especial interest from a 

 distributional point of view, since they indicate how the Opossums of 

 America may have been connected with the Australian Marsupials. 

 These forms were originally referred to Didelphys, but have been 

 subsequently described as Peratherium and Amphiperatherium. The 

 characters of the molar teeth on which these genera are based do 

 not appear to be sufficiently important to justify their separation 

 from Didelphys. Allied forms occur in the Tertiaries of North 

 America, which were originally described under the name of Her- 

 petotherium, but have been subsequently referred to Peratherium. 

 Remains of many of the existing species of Opossum are found in 

 a fossil condition in the Pleistocene cave-deposits of Brazil. 



