The Collaborative International Dictionary of English v.0.48:

Opossum \O*pos"sum\, n. [Of N. American Indian origin.] (Zool.)
   Any American marsupial of the genera Didelphys and
   Chironectes; called also possum. The common species of
   the United States is Didelphys Virginiana.
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   Note: Several related species are found in South America. The
         water opossum of Brazil (Chironectes variegatus),
         which has the hind feet, webbed, is provided with a
         marsupial pouch and with cheek pouches. It is called
         also yapock.
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   Opossum mouse. (Zool.) See Flying mouse, under Flying.


   Opossum shrimp (Zool.), any schizopod crustacean of the
      genus Mysis and allied genera. See Schizopoda.
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The Collaborative International Dictionary of English v.0.48:

Yapock \Ya"pock\ (?; 277), n. [Probably from the river Oyapok,
   between French Guiana and Brazil.] (Zool.)
   A South American aquatic opossum (Chironectes variegatus)
   found in Guiana and Brazil. Its hind feet are webbed, and its
   fore feet do not have an opposable thumb for climbing. Called
   also water opossum. [Written also yapack.]
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