The Collaborative International Dictionary of English v.0.48:

Didelphys \Di*del"phys\, n. [NL. See Didelphia.] (Zool.)
   Formerly, any marsupial; but the term is now restricted to an
   American genus which includes the opossums, of which there
   are many species. See Opossum. [Written also Didelphis.]
   See Illustration in Appendix. --Cuvier.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Didelphis
    n 1: type genus of the family Didelphidae [syn: Didelphis,
         genus Didelphis]