The Collaborative International Dictionary of English v.0.48:

Water flounder \Wa"ter floun"der\ (Zool.)
   The windowpane (Pleuronectes maculatus). [Local, U. S.]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Windowpane \Win"dow*pane`\, n.
   1. (Arch.) See Pane, n., (3)
      b . [In this sense, written also window pane.]
        [1913 Webster]

   2. (Zool.) A thin, spotted American turbot (Pleuronectes
      maculatus) remarkable for its translucency. It is not
      valued as a food fish. Called also spotted turbot,
      daylight, spotted sand flounder, and water flounder.
      [1913 Webster]