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]