Bivalve mollusks are saltwater creatures with two shells, such as mussels (moules), clams (palourdes), oysters (huîtres), and scallops (coquilles Saint-Jacques). Univalves, which include snails (escargots) and winkles (bigorneaux), are one-shelled inhabitants of either fresh water, saltwater, or land.
Of all commonly available mollusks, scallops are the only ones always sold shucked. The tiniest available are calico scallops, and the largest are sea scallops. Bay scallops fall somewhere in between. “Day-boat” scallops are now sold across the country, but this term signifies only that they are freshly harvested and brought to market in a day. In the Pacific Northwest, pink scallops are sold live in the shell, but they are not usually shipped outside the area. Occasionally, the finest purveyors now have scallops in the shell along with their roe.