Everyone who has spent a little time in naturalising on the shore, has noticed how often you may find univalve shells, such as those of the whelk and periwinkle, with the top of the shell knocked off. This is nearly always the case with the dead shells that you find strewn along the tide-line; and after a storm, on a rocky coast, you may find shells that still contain the living tenant, in the same sad condition. And you may also meet not infrequently with shells, dead or living, that bear evidence of the owners' efforts to repair them after an accident to the spire. A piece has been broken, and you find it cemented on again by a patch of shell, serviceable no doubt to the owner, but crooked and unsightly in appearance. Now there is a very common shell, the little yellow periwinkle, which has practically done away with its spire, the coils of the shell being so curved that the earlier part of the spire does not project beyond the later-formed coils, and the whole shell has a rounded outline. This little creature lives on the long seaweeds which grow at low-water mark or near it; and when the sea is rough it is obviously liable to be dashed from its foothold on the seaweed and flung violently down, as the huge seaweeds sway about in the shallow waves. We may easily satisfy ourselves that this is an accident that frequently happens, by examining the shore when the tide is going out, on some stormy spring or autumn day. Numbers of the yellow periwinkles are then to be found crawling on the sand, and striving to regain their place in the seaweedy rocks as soon as possible. On a calm day you will rarely see one crawling on sand above low-water mark, for it is a place they do not choose by preference; those that are to be found there on the stormy day have lost their foothold, and have been washed about by the tide. Had they, like some other kinds of periwinkle, a sharp spire, how many would be the casualties under these circumstances! But as it is, you do not see a single specimen with a broken top: the rounded spire is an adaptation to circumstances, required for the protection of the tenant of the shell. (See Fig. 2.)