Page:Natural History, Mollusca.djvu/214

202 by the common Periwinkle, in which each of the infant animals is seen enclosed in its proper globule, and covered already with a shell of a single whorl.

The Periwinkle (L. littorea) is subject to much variation in form, colour, and markings; it is generally about an inch in height, globose, very thick, with the margin thin; the colour is russet brown, or olive, sometimes yellowish, with spiral bands of black. The ground colour occasionally becomes a rich orange or scarlet, with or without black bands; and these varieties are very handsome.

This is a group of fresh-water Mollusks, so closely resembling the Periwinkles in many of their characters as to have been confounded with them by Cuvier and other zoologists. Those characters are as follows:—

The shell is conical, spiral, thin in texture, covered with an olive-coloured skin; the orifice is ovate, entire, but angular behind. The animal has a muzzle-shaped head, and long, slender tentacles, with eyes seated at the outer side of their bases; the gills are always enclosed in the breathing cavity. An operculum is always present, generally horny, but sometimes shelly, formed of concentric