Hydrocharis Morsus-ran[ae]

from The Collaborative International Dictionary of English v.0.48

frogbit \frog"bit`\, frog's-bit \frog's-bit\, n. (Bot.)
      (a) A European plant ({Hydrocharis Morsus-ran[ae]}),
          floating on still water and propagating itself by
          runners. It has roundish heart-shaped leaves and small
          white flowers.
      (b) An American plant ({Limnobium Spongia}), with similar
          habits.
          [1913 Webster]