half shell

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

A phrase commonly used to describe raw oysters served on the bottom shell only, usually on a plate of crushed ice or, in the case of cooked dishes such as oysters rockefeller, on a bed of rock salt. Some oyster lovers eat these fresh oysters without any condiments, sipping the oyster liquor from its bottom shell. Others adorn theirs with lemon juice, horseradish, tabasco sauce, cocktail sauce, ketchup or vinegar.