The phylum Mollusca is divided into seven classes. Basic features of a typical gastropod shell are described. Generally mollusk shells consist of calcium carbonate (CaCO₃) which may occur as aragonite, as calcite, or as layers of both. Essential tools for proper identification include access to correctly identified material. Mollusk specimens encountered in archaeological studies often are in poor condition or fragmented; therefore, identification should be confirmed by experts. A list of basic reference books is appended.