MARINE MOLLUSCS 227 



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 some- 

 times 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. 



It will be noticed that in the above general description of uni- 

 valve shells we have introduced a number of technical terms 

 which are printed in italics, and this we have done advisedly, for 

 the employment of these terms is a very great convenience when 



