At the lower or anterior part, sometimes called the _columella_, there are in many instances flattened, laminated folds; these are particularly conspicuous in the genera Cymba and Melo, where, being obliquely spiral and laminar, they are extremely elegant, presenting to the eye graduated repetitions of the line of beauty. In other cases, as in the Turbinellæ, they are more horizontal and thickened. In some cases the columella is swelled into a varicose mass; as in Oliva, Ancillaria, &c.; it is then described as _tumid_ or varicose. It is sometimes _tortuous_, and sometimes straight, and is susceptible of many variations, too minute and particular to be described in this part of the work. COLUMELLAR LIPS. [Illustration: Fig. 60, Melo, _obliquely plaited_; 61, Turbinellus, _horizontally plaited_; 62, Ancillaria, _varicose_, _tortuous_; 63, Natica, _straight_.] OPERCULUM. The aperture of many species of shells remains constantly open; but in a great number of species it is occasionally closed, whenever the animal is retracted within the shell, by a calcareous or horny piece called the operculum. This must be distinguished in the first instance from another kind of calcareous covering, which in some univalve shells serves to close the aperture during a certain portion of the year. This piece, named the _epiphragm_, although hardened and shelly in appearance, is no real part of the animal or of the shell; being only a secretion temporarily hardened, for the purpose of defending the animal from external influences during the _hibernating_ or _torpid_ season, to be dissolved when that season is at an end. On examining this piece, it will be observed that it is not formed in regular layers like the rest of the shell; while the true operculum is of a regularly laminated structure, having a nucleus and receiving obliquely deposited additions, either in a lateral spiral or concentric direction. It is attached to the posterior part of the foot on the upper surface; and when the animal retires within its shell, that part of the foot enters last, drawing the operculum after it, and thus closing the aperture. The opercula of various shells differ in the first place as to their chemical composition. They are _calcareous_ when formed principally of calcareous matter, like the rest of the shell, as in Neritina, Nerita, and some others. They are _corneo-calcareous_, when upon an internal lamina of horny consistency there is a thickened layer of shelly matter. This is the case with shells of the genus Turbo and Phasianella, which are on this account distinguished from those of the genus Trochus; the opercula of the latter being horny or _corneus_. The size of the operculum is distinguished by comparison with the rest of the shell; thus, those of Strombus, Cassis, &c. are small; while those of Cyclostoma and others are large, filling up the cavity at its outer edge.