246 VARIOUS FORMS OF THE SPIRAL chap. 



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^ 

 Vag inula) . 



Varieties of the Spiral. — Almost every conceivable modifi- 



FiG. 150. — Examples of shells with A, a flattened 

 spire {Polygyratia) ; B, a globose spire {Nat- 

 ica) ; C, a greatly produced spire (Terebra). 



C 



cation of the spiral occurs, from the type represented by G-ena, 

 Haliotis^ Sigaretus, and Lamellaria, in which the spire is practi- 

 cally confined to the few apical whorls, with the body-whorl 

 inordinately large in proportion, to a multispiral form like 

 Terehra^ with about twenty whorls, very gradually increasing 

 in size. 



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, peri- 

 winkle, etc. 



