Source : Webster's Revised Unabridged Dictionary (1913)

Soft-shell \Soft"-shell`\, Soft-shelled \Soft"-shelled`\, a.
   Having a soft or fragile shell.

   {Soft-shell clam} (Zo["o]l.), the long clam. See {Mya}.

   {Soft-shelled crab}. (Zo["o]l.) See the Note under {Crab}, 1.
      

   {Soft-shelled turtle}. (Zo["o]l.) Same as {Soft tortoise},
      under {Soft}.

Source : WordNet®

soft-shell clam
     n 1: a clam that is usually steamed in the shell [syn: {steamer},
           {steamer clam}, {long-neck clam}]
     2: an edible clam with thin oval-shaped shell found in coastal
        regions of the United States and Europe [syn: {steamer}, {steamer
        clam}, {long-neck clam}, {Mya arenaria}]