Varices are caused by periodical rests or stoppages in the growth of the shell, when the edge of the aperture thickens, and renders the shell as complete as when full grown. Again, after an interval, another check takes place, and another thickened edge is formed, and so on in succession, until the animal arrives at maturity, and the shell is full-grown. The thickened edges successively forming the aperture, remain visible on the outside, through all the subsequent stages. When these rests take place at frequent periods, the varices will of course be numerous as in Harpa and Scalaria. They occur at regular or irregular distances, varying in shape and other characters. When the varices occur at regular intervals, and form a connected ridge from whorl to whorl up the spire, they are said to be _continuous_, as in Ranella; when on the contrary, the varix on one whorl does not come in contact with that on the other, they are described as _discontinuous_. In order to distinguish a regular varix from a mere external ridge, it will be sufficient to notice whether its edge overlaps the external surface, and whether it resembles the open edge of the aperture, which true varices do. [Illustration: Fig. 40, _numerous_; 41, _few, continuous_; 42, _few, discontinuous_.] _Aperture._ The aperture or opening of the spiral tube, was formerly described as the mouth; a term calculated to convey an erroneous impression, when applied to a part of the shell which has no correspondence with the mouth of the animal. The word _aperture_ is used by modern writers in a general sense, including the cavity, its edge, and the canals. The cavity itself is distinguished in various shells as to its shape, which depends much upon the degree of modification produced by the last whorl. In some cases, as in Cyclostoma, where the aperture stands apart from the last whorl, the shape is round, or nearly so. The Scalaria presents a good example of this. In others, where the inner edge or lip, wrapping over the body whorl is nearly straight, the aperture is _semi-lunar_, or half-moon shaped: this is remarkable in the "_Neritacea_" of Lamarck, named, on that account, "_hemi-cyclostomata_" by De Blainville. In a great number of instances, the lower part of the body whorl enters obliquely into the upper part of the aperture, the result being a _pyriform_, or pear-shaped opening. The aperture is described as _long_ when it is largest in the direction of the axis, and _wide_, in the contrary case. The _anterior_ is the part at the greatest distance from the apex, and the body whorl; the _posterior_, the part nearest to the apex. Thus some apertures are described as _posteriorly contracted_ and _anteriorly widened_, or the reverse. A _linear_ aperture is one contracted in its whole length, as in Cypræa. When the whorls are angulated, a _trigonal_ aperture is the result, as in many species of Trochus. Some are _transversely oval_, that is in an opposite direction to