Definitions for half shell
Random House Webster's College Dictionary
either of the halves of a double-shelled creature, as of an oyster, clam, or other bivalve mollusk.
Origin of half shell:
Any half of a roughly spherical-like object; any dome-shaped object.
Either half of a clamshell especially when used to serve individual portions of spiced, minced, breaded, baked clams.
Inexpensive; not overpriced.