Page:Natural History, Mollusca.djvu/166

 genus, on the surface of which it forms a flat disk, exactly agreeing in size with the circumference of its own shell. To form these depressed disks, (of which there are so generally two on each larger Patella, one on each side of the apex, as almost to form a character of the species,) and to assist in the increase of its size, the animal appears also to absorb the coralline or other similar substances with which the larger shells are abundantly covered.

COMMON LIMPET.

But we need not wander to the southern hemisphere for illustrations of this power. The most familiar shell-fish of our shores, the common Limpet (P. vulgata), will afford one equally good. Who has not seen the oval pits, sometimes but just discernible, at others sunk to the depth of an eighth of an inch or more, on the rocks of our coast, each accurately corresponding in shape and dimensions with a Limpet which inhabits it? I have wondered at them many times, not being then aware of the habits which have been ascribed to these animals, of wandering away from these pits, (which they have chosen for a home,) and of returning to them regularly again.