planorbis

from The Collaborative International Dictionary of English v.0.48

Planorbis \Pla*nor"bis\, n. [NL., fr. L. planus flat + orbis a
   circle.] (Zool.)
   Any fresh-water air-breathing mollusk belonging to
   {Planorbis} and other allied genera, having shells of a
   discoidal form.
   [1913 Webster]