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