half shell

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Either of the halves of a shell of a bivalve, such as an oyster.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective rare, colloquial, especially with "on the" Inexpensive; not overpriced.
• noun physics, figuratively Any half of a roughly spherical-like object; any dome-shaped object.
• noun of food, especially with "on the" Either half of a clamshell especially when used to serve individual portions of spiced, minced, breaded, baked clams.