The Collaborative International Dictionary of English v.0.48:

Soft-shell \Soft"-shell`\, Soft-shelled \Soft"-shelled`\, a.
   Having a soft or fragile shell.
   [1913 Webster]

   Soft-shell clam (Zool.), the long clam. See Mya.

   Soft-shelled crab. (Zool.) See the Note under Crab, 1.

   Soft-shelled turtle. (Zool.) Same as Soft tortoise, under
      Soft.
      [1913 Webster]

WordNet (r) 3.0 (2006):

soft-shelled turtle
    n 1: voracious aquatic turtle with a flat flexible shell covered
         by a leathery skin; can inflict painful bites [syn: soft-
         shelled turtle, pancake turtle]