Washington clams of the genus Saxidomus, are esteemed as food on the Pacific coast of N. America. They are large bivalves with thick shells, which bear numerous and finely spaced concentric lines but no radial ones. The northern form, S. giganteus, has a range from Alaska down to San Francisco. It may measure 11 cm (just over 4"). Its specific name seems to have been chosen without adequate reflection, since the other species, S. nuttalli, is much larger: up to 17 cm (nearly 7"). This is a purely Californian species, with a range from Humboldt Bay to San Quintin Bay in Baja California, and may be distinguished from its smaller relation by purple markings on the inside of the shells. The fishery for it is extensive. The two species are marketed together and share the same common names, which include butter clam and money clam as well as Washington clam.