=The Univalve Shell.=--In univalve Mollusca the normal form of the shell is an elongated cone twisted into a spiral form round an axis, the spiral ascending to the left. Probably the original form of the shell was a simple cone, which covered the vital parts like a tent. As these parts tended to increase in size, their position on the dorsal side of the animal caused them gradually to fall over, drawing the shell with them. The result of these two forces combined, the increasing size of the visceral hump, and its tendency to pull the shell over with it, probably resulted in the conversion of the conical into the spiral shell, which gradually came to envelop the whole animal. Where the visceral hump, instead of increasing in size, became flattened, the conical shape of the shell may have been modified into a simple elliptical plate (_e.g._ _Limax_), the nucleus representing the apex of the cone. In extreme cases even this plate dwindles to a few calcareous granules, or disappears altogether (_Arion_, _Vaginula_). =Varieties of the Spiral.=--Almost every conceivable modification of the spiral occurs, from the type represented by _Gena_, _Haliotis_, _Sigaretus_, and _Lamellaria_, in which the spire is practically confined to the few apical whorls, with the body-whorl inordinately large in proportion, to a multispiral form like _Terebra_, with about twenty whorls, very gradually increasing in size. [Illustration: FIG. 150.--Examples of shells with =A=, a flattened spire (_Polygyratia_); =B=, a globose spire (_Natica_); =C=, a greatly produced spire (_Terebra_).] As a rule, the spire is more or less obliquely coiled round the axis, each whorl being partially covered, and therefore hidden by, its immediate successor, while the size of the whorls, and therefore the diameter of the spire as a whole, increases somewhat rapidly. The effect of this is to produce the elevated spire, the shell of six to ten whorls, and the wide aperture, of the normal type of mollusc, the whelk, snail, periwinkle, etc. Sometimes, however, the coil of the whorls, instead of being oblique, tends to become horizontal to the axis, and thus we have another series of gradations of form, from the excessively produced spire of _Terebra_ to the flattened disc of _Planorbis_, _Polygyratia_, _Euomphalus_, and _Ammonites_. The shell of many species of _Conus_ practically belongs to the latter type, each whorl folding so closely over its predecessor that the spiral nature of the shell is not perceived until it is looked at at right angles to the spire. [Illustration: FIG. 151.--Examples of shells with disconnected whorls; =A=, _Cyathopoma cornu_ Mf., Philippines; =B=, _Cylindrella hystrix_ Wright, Cuba. (Both × 4.)] [Illustration: FIG. 152.--Example of a shell whose apical whorls alone are coiled, and the remainder produced in a regular curve. (_Cyclosurus Mariei_ Morel., Mayotte.)]