Source : Webster's Revised Unabridged Dictionary (1913)

Yapock \Ya"pock\ (?; 277), n. [Probably from the river Oyapok,
   between French Guiana and Brazil.] (Zo["o]l.)
   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}.]