The Periwinkle, Littorina littorea (L.), is one of the most abundant gastropods on the shore, and can live under widely varying conditions. It is found on clean, alga-covered rocks, among small stones, on gravel, on soft mud, and rarely even on sand. It ranges from about high water of neap tides to extreme low water of spring tides, and in some places even slightly below this, although in most localities it is restricted to a much smaller part of the tidal zone. It occurs also from shores with high wave-exposure to sheltered and even polluted estuaries, where it is tolerant of considerable lowering of the salinity. The material for the present study was collected at intervals throughout 1934—1936 from three localities near Plymouth, the details of which are as follows.
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