The mouth of the shell is of different forms in different species, but in the herbivorous kinds it is usually simple, while in the carnivorous species it is notched or produced. The edge of the mouth (_peristome_) is formed by an _outer lip_ which is usually sharp in young shells and either thickened, reflected (turned outward), or inflected (turned inward) in adults; also it may be considerably expanded, or ornamented by a fringed margin. The _inner lip_ is that side of the peristome adjacent to the central pillar of the shell. If we examine the external surface of several different shells, we find that they are usually more or less distinctly furrowed or sculptured, and that they are often marked by lines or bands of a colour different from that of the ground tint. These furrows, lines, or bands sometimes pass directly from the apex, across the various whorls, to the base of the shell, in which case they are said to be _longitudinal_. If they follow the course of the whorls, they are described as spiral; and if parallel with the peristome, so that they mark the former positions of the mouth of the shell, thus denoting the _lines of growth_, they are said to be _transverse_. Most univalve shells are covered with epidermis, but in some instances the animal, when extended, surrounds the exterior of the shell with its mantle, as do the cowries, and then the outside of the shell is always glazed. Other species keep their shells covered with the mantle, and in these the shell is always colourless. The body of the head-bearing mollusc is attached to the shell internally by one or more muscles, and if we examine the interior surface we are generally able to distinguish the impressions or scars denoting the points of attachment. The reader will have observed that the periwinkle, whelk, and other univalves close their shells by a kind of lid when they retract their bodies. This lid is called the _operculum_, and is constructed of a horny material, often more or less calcified on the exterior, and is attached to the hinder part of the foot. It sometimes fits accurately into the mouth of the shell, but in some species it only partially closes the aperture. The operculum, like the shell itself, often exhibits distinct lines of growth which display the manner in which it was built up. If these lines are concentric we know that the operculum grew by additions on all sides; but if its nucleus is at one edge, and the lines of growth widest apart at the opposite side, the growth must have taken place on one side only. Some, even, are of a spiral form, denoting that the additions were made continuously at one edge, and such opercula may be right-handed or left-handed spirals.