The Collaborative International Dictionary of English v.0.48:

Flounder \Floun"der\, n. [Cf. Sw. flundra; akin to Dan. flynder,
   Icel. fly?ra, G. flunder, and perh. to E. flounder, v.i.]
   1. (Zool.) A flatfish of the family Pleuronectid[ae], of
      many species.
      [1913 Webster]

   Note: The common English flounder is Pleuronectes flesus.
         There are several common American species used as food;
         as the smooth flounder (P. glabra); the rough or
         winter flounder (P. Americanus); the summer flounder,
         or plaice (Paralichthys dentatus), Atlantic coast;
         and the starry flounder (Pleuronectes stellatus).
         [1913 Webster]

   2. (Bootmaking) A tool used in crimping boot fronts.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Plaice \Plaice\, n. [F. plaise, plais, prob. fr. L. platessa
   flatish, plaice. See Place.] (Zool.)
   (a) A European food fish (Pleuronectes platessa), allied to
       the flounder, and growing to the weight of eight or ten
       pounds or more.
   (b) A large American flounder (Paralichthys dentatus;
       called also brail, puckermouth, and summer
       flounder. The name is sometimes applied to other allied
       species. [Written also plaise.]
       [1913 Webster]

   Plaice mouth, a mouth like that of a plaice; a small or wry
      mouth. [R.] --B. Jonson.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Paralichthys dentatus
    n 1: flounder of eastern coast of North America [syn: summer
         flounder, Paralichthys dentatus]