The Collaborative International Dictionary of English v.0.48:

Potamian \Po*ta"mi*an\, n. [Gr. ? river.] (Zool.)
   A river tortoise; one of a group of tortoises (Potamites,
   or Trionychoidea) having a soft shell, webbed feet, and a
   sharp beak. See Trionyx.
   [1913 Webster]