the axis, and others _longitudinally oval_. When the whorls are formed with two outer angles, a somewhat quadrated aperture is formed. There are other variations too numerous to mention. _Apertures._ [Illustration: Fig. 43, Helicina, _semilunar_; 44, Pirena, _pyriform_; 45, Cypræa, _linear_; 46, Trochus, _trigonal_; 47, Cyclostoma, _rounded_; 48, Chilina, _posteriorly contracted_; 49, Stomatia, _transversely oval_; 50, Murex, _longitudinally oval_.] The entire edge of the aperture described generally, is named the Peritrême, but this term can only be conveniently applied in cases where, in some at least of its characters, it is the same all round, so that one descriptive term is applicable to the whole. As, however, this is of rare occurrence, it is found convenient in descriptions to separate the rim from the outer lip. In a great number of instances, this is done naturally, by a canal, or notch at the anterior or lower extremity, and by the posterior union of that part which overlays the body whorl with the other portion. At these two points the outer and inner lips separate from each other: we therefore describe the _Canals of the Aperture._ When there is neither notch nor canal, anteriorly or posteriorly, interrupting the edge of the aperture, it is described as entire. When there is a notch or sinus at the anterior extremity, it is said to be _emarginated_. When the edge of this notch is expanded, and drawn out in the form of a beak, it is said to be _canaliferous_, or to have a _canal_. When, in addition to this, the lips are thickened and contracted posteriorly near their junction, and drawn out so as to form a groove, it is said to be _bi-canaliculated_, or to have two canals. The _anterior canal_ is said to be long or short, according to the proportion which it may bear to the rest of the shell. Thus the canal of Ranella ranina (fig. 393 in the plates), may be described as _short_; while that of Murex haustellum, (fig. 396, pl.) is _long_. When it is wide near the aperture, and becomes gradually contracted towards its termination, it is said to be _tapering_, as in Pyrula (fig. 388, pl.); when the termination is sudden, it is described as _truncated_. If, on placing the shell upon a plane, with the aperture downwards, the canal is seen to rise upwards, it is _recurved_. In Buccinum and Nassa it is turned suddenly over the back, and forms a short, curved elevation; it is then described as _recurved_ and _varicose_. If the edges meet, so as to form a tube, it is said to be closed, as in some species of Murex and Typhis. The posterior canal is, in some cases, _free_, or standing out from the spire, as in some species of Ranellæ; while in others it is _decumbent_, running up the sides of the spire, as in Rostellaria (fig. 402, pl.). _Canals._ [Illustration: Fig. 51, Fasciolaria, _truncated_; 52, Nassa, _recurved_, _varicose_; 53, Cerithium, _recurved_; 54, Typhis, _closed_.] _Lips, or edges of the Aperture._