=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. [Illustration: 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_. [Illustration: FIG. 175.--Anal slit in _Pleurotoma_.]