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Related to butterfly shell: Tortoise Shell Butterfly
n., pl. -nas.
1. Also called butterfly shell. a small clam, Donax variabilis, having fanlike bands of various hues and common in intertidal zones of the E and S U.S. coasts: the paired empty shells often spread in a butterfly shape.
2. any similar clam.
3. a soft whitish rock made up of fragments of marine shells and coral, used as a building material.