246 



VARIOUS FORMS OF THE SPIRAL 



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 hmnp, 

 and its tendency to pull the shell over with it, prolmhly resulted 

 in the conversion of the conical into the spiral shell, which 

 gradually came to envelop tlie 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 modifi- 



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

 spire (Polygyratia) ; B, a globose spire {Natica) ; 

 C, a greatly produced spire (Tcrehra). 



cation of the spiral occm'S, from the type represented by Gcna, 

 Haliotis, Sigaretus, and LameUaria, in wldcli the spire is practi- 

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

 inordinately large in proportion, to a multispiral form like 

 Tcrehra, witli about twenty whorls, very gradually increasing in 

 size. 



As a rule, the spire is more or less ol)liquely 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 

 apertm-e, of the normal type of mollusc, the whelk, snail, peri- 

 winkle, etc. 



