Terms employed to denote various Parts of the Univalve Shell.—The apex is the extreme top of the spire, and generally consists of the embryonic shell, which may often be recognised by its entire want of sculpture. When the embryonic shell happens to be large, the apex is often mammillated, e.g. in Fusus, Neptunea, and some Turbinella; in the Pyramidellidae it is sinistral.
The suture is the line of junction between any two successive whorls. It may be deep, and even channelled, or very shallow, as in Fig. [150] B (p. 246).
The spire is the whole series of whorls except the last or body whorl. A whorl is a single revolution of the spiral cone round the axis. The spire may be subulate (as in Terebra, Fig. [150] C), turreted (Scalaria), depressed (Polygyratia, Fig. [150] A), conical (Trochus), globose (Ampullaria, Natica, Fig. [150] B), with almost all conceivable gradations between these types. The number of whorls is best counted by placing the shell mouth downwards, and reckoning one for every suture that occurs between the extreme anterior point of the shell and the apex.
Fig. 174.—Illustrating the technical terms applied to the various parts of a univalve shell.
The mouth or aperture may be (a) entire, as in Helix, Natica, Ampullaria, when its peristome or margin is not interrupted by any notch or canal, or (b) prolonged at its anterior and sometimes also at its posterior end into a canal. The anterior canal serves as a protection to the siphon,[347] the posterior canal is mainly anal in function, and corresponds, in part, to the hole of Fissurella, the slit in Pleurotoma and Emarginula, and the row of holes in Haliotis. The mouth presents every variety of shape, from the perfect circle in Cyclostoma and Trochus, to the narrow and prolonged slit in Conus and Oliva.
Fig. 175.—Anal slit in Pleurotoma.
The right margin of the mouth (the left, in sinistral shells) is termed the outer lip or labrum, the left margin the inner lip, labium, or columella lip.[348] In young shells the outer lip is usually thin and unfinished, while in the adult it is generally thickened into a rib, or furnished with more or less prominent teeth, or given an inward or outward curve. In some genera, especially the Strombidae, the outer lip of the adult develops long finger-like processes, which sometimes attain an extraordinary size (chap. [xiv].). As growth proceeds, these marginal teeth and ribs are either dissolved and disappear, or are permanently incorporated, in the shape of varices, with the framework of the shell. Some shells, e.g. Natica, Turritella, Actaeon, have a permanently unfinished outer lip, even in the adult stage. The columella lip varies in shape with the mouth as a whole; thus it may be straight, as in Conus, or excavated, as in Sigaretus, Struthiolaria, and Bulla. Frequently it is continued by part of the body whorl, as in Ficula, Dolium, and Fasciolaria.
