periwinkle, any of a group of marine gastropod mollusks having conical, spiral shells. Periwinkles feed on algae and seaweed. They are found at the water's edge; out of water, they resist drying by closing themselves into the shell with a horny plate. The edible European species, called the common periwinkle, has become well established on the Atlantic coast of North America. About 12 other species are found on rocky beaches of both the Atlantic and the Pacific coasts. Periwinkles are classified in the phylum Mollusca, class Gastropoda, order Mesogastropoda, family Littorinidae, genus Littorina.