Trapa bicornis

from WordNet (r) 3.0 (2006)

Trapa bicornis
    n 1: water chestnut whose spiny fruit has two rather than 4
         prongs [syn: {ling}, {ling ko}, {Trapa bicornis}]

from The Collaborative International Dictionary of English v.0.48

Water chestnut \Wa"ter chest"nut\ (Bot.)
   The fruit of {Trapa natans} and {Trapa bicornis}, Old World
   water plants bearing edible nutlike fruits armed with several
   hard and sharp points; also, the plant itself; -- called also
   {water caltrop}.
   [1913 Webster]