Periwinkles have solidly built shells with typically sharp round protuberances. The outer lip of the shell is sharp-edged and thin, while the mollusc has a horny operculum. The species shown here is larger than many (4 cm.) and was found in Japan. They are found in intertidal locations widely distributed in most parts of the world, and often in swarms on harbor walls and other rocky surfaces. Some, however, live in mangroves.
The range of conditions in which periwinkles live can be surprising. Some, like
, have adapted to harsh dry, hot conditions which occur in places that may be wet only once or twice a month during the highest of the high tides
Interestingly, several families in the superfamily, Littorinoidea, have further adapted to heavily muddy water by developing the capacity to breath air directly (see, e.g.,