What is "half shell"

a. (context rare colloquial especially with "on the" English) Inexpensive; not overpriced. alt. (context rare colloquial especially with "on the" English) Inexpensive; not overpriced. n. 1 (context physics figuratively English) Any half of a roughly spherical-like object; any dome-shaped object. 2 (context of food especially with "on the" English) Either half of a clamshell especially when used to serve individual portions of spiced, minced, breaded, baked clams.