Genus Littorina.
The shell in this common and well-known genus is spiral with but few whorls, generally more or less oval in form, and thick and solid in substance. The spire is sometimes pointed, as in the common Periwinkle (L. littorea); sometimes obtuse or round, as in the equally common Yellow Winkle (L. neritoides). The aperture is round and entire; the outer lip is sharp-edged, thickened within. The operculum is horny and elastic; its spire consists of a few turns, rapidly enlarging, with a central nucleus.
In most of our seaport towns, and in many of our inland cities, the Periwinkle is sufficiently familiar, from its being so commonly sold by measure as human food. The animals are found by thousands on rocks at low-water, or on the mud left exposed by the receding tide in harbours and estuaries; they are collected by the children of fishermen, boiled, and hawked about the streets at a low price. They are eaten not infrequently by persons above the lowest grade of society, not from necessity, but from choice; though to most uneducated palates they are coarse, tough, and indigestible.
The Periwinkles are able to bear long-continued exposure to the air with impunity. The species just mentioned may be observed adhering to the rocks by hundreds under a hot sun, and that for hours together; but a smaller kind (L. rudis),—which may be recognised by its being frequently found white, pale-green, yellow, and orange in colour,—habitually resides in hollows of rocks that are elevated many feet above the range of high-
