DIDELPHVIDE 135 
fourth or Jficowreine 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 
Fia. 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 hased 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. 
