[Illustration] The length is measured from the apex, to that part of the aperture _a_ (fig. 24), at the greatest distance from it. The _breadth_ is in the opposite direction. The _anterior_, or front part of the aperture, is marked _a_, where the head of the animal protrudes. _Spire of non-symmetrical Univalves._ [Illustration: Fig. 25, _obtuse_; 26, _acute_; 27, 28, _decollated_; 29, _concave_; 30, _papillary_; 31, _mammellated_; 32, _discoidal_.] In counting the whorls of which the spire consists, we commence at the apex, and reckon downwards to the last, or body whorl. The spire is described as being long or short in relation to the aperture: in which case, all that is above the aperture is measured with the spire. Its apex requires particular notice, as the character of the whole shell frequently depends upon the particulars observable in this part. It is sometimes _obtuse_, or blunt; sometimes _acute_, or sharp. In the Cones it is frequently flat, and in Planorbis it is concave. It is sometimes of a different structure from the rest of the shell, retaining the horny and transparent appearance which characterized it when the animal was first hatched. The Tritons present an instance of this, although it is not always observable, owing to the tenderness of the substances which causes it to break or fall away in many specimens. A very remarkable instance also occurs in Bulinus decollatus (cut, fig. 27, 28), so named, because the apex, to the depth of several whorls, falls off, and the shell is _decollated_. In this, and many more instances, among Pupæform land shells, the occurrence of this circumstance seems to be by no means rare or accidental, a provision having been made for filling up the opening by a septum. A _papillary apex_ is one which is swelled at the extremity into a little rounded nob, or nipple; and a _mammellated_ apex is one which is rounded out more fully into the shape of a teat. _Whorls._ The spire is described as consisting of _numerous_ or _few_ whorls, and sometimes the number of them is particularly stated. A whorl consists of one turn of the spiral cone. The whorls are described as _flattened_, when the sides are not bulged out so as to cause the outline of the spire to deviate considerably from straightness: when the contrary is the case, the whorls are said to be _ventricose_, and either _rounded_ or _angulated_. The degree of rapidity with which the whorls become enlarged presents an important source of distinction. The _suture_, or seam, which separates one whorl from another is also noticed as being _distinct_ or otherwise; _canaliculated_, or grooved; or covered by an enamel, which in some instances is swelled into a ridge or _tumid_. [Illustration: Fig. 33, _few_; 34, _numerous_; 35, _rounded_, _ventricose_; 36, _angular_, _ventricose_; 37, _flattened_.] _Suture._ [Illustration: Fig. 38, _canaliculated_; 39, _enamelled_.] _Varices._