One minor correction: The shell of most brachiopods, including all of the
hinged ones (Articulata) is composed of calcite, the same form of calcium
carbonate as in oysters and scallops -- though the crystals are arranged
differently. Terebratalia is an articulate brachiopod. However, most of the
unhinged brachiopods (Inarticulata or inarticulates) have a shell composed
of apatite (calcium phosphate hydroxide). The best-known inarticulate
brachiopod is Lingula. Apatite is less soluble than calcite in most acids,
and will not fizz when acid is dropped on the shell, because the reaction
does not yield a gas.

Andrew K. Rindsberg
Geological Survey of Alabama