The littorinas fairly swarm in favorable localities upon all shore stations. In Maine and Massachusetts the bold, rocky coast furnishes a home for several species. Often the rocks at low tide are black with them; the algæ that cling in wet masses to the exposed rocks are alive with them. One cannot walk about in such localities without crushing hundreds of specimens. Sometimes they will be found clinging in clusters upon the piling of old wharves, or crawling about the bottom at or about the low-tide mark. The best specimens of /Littorina/ are found in stations where they are bathed twice a day by pure, uncontaminated sea-water; those living near the mouths of streams, or where the water is brackish or impure, are usually small and degenerate. They are vegetable feeders, and have received the common name of "periwinkles." In Great Britain they are used among the poorer classes for food. The animal has a short, broad muzzle, and eyes at the outer bases of the tentacles. The foot is longitudinally grooved, and there is a rudimentary siphonal fold in the mantle. The shells are turbinated, usually heavy, few-whorled, and with a round aperture. [Illustration: /Littorina litorea./] ^L. litorea.^ This is supposed to be an importation from the Old World—to have come over by way of Iceland and Greenland, and then to have migrated down the Labrador coast. For many years Cape Cod formed a barrier to its advance, but now the species is abundant at Newport, and is reported at New York. It occurs on the Maine coast in astonishing numbers, living in vast colonies on the rocks exposed at low tide. The shell is thick, imperforate (no umbilicus), and usually has flat, spiral ribs. The columella is broad and white; the lip thin and black. The general color varies from black to olive or to dingy gray—sometimes reddish. The operculum is corneous, with the nucleus near the outer edge. Despite the variableness of this very common shell (the variations being chiefly in the height of the spire), it has certain unmistakable characteristics which, once seen, will enable the collector to determine it at once. [Illustration: /Littorina rudis./]