242 THE MECHANICAL FUNCTIONS. 



shell in the progress of its increase. The Pa- 

 tella, or limpet, is an example of this form of 

 structure. 



But in by far the greater number of mollusca 

 which inhabit univalve shells, the formation and 

 deposition of the earthy material does not pro- 

 ceed equally on all sides, as happens in the pa- 

 tella. If the increase take place in front only, 

 that is, in the fore part of the mantle, the con- 

 tinual deflexion thence arising necessarily gives 

 the shell a spiral form, the coils being simply 

 in one plane. This is the case in the Planorbis, 

 (Fig. 105) the Spirula and the Nautilus. Most 

 commonly, however, as in the Buccinum, and 

 Achatina, (Fig. 108) the deposit of shell takes 

 place laterally, and more on one side than on the 

 other ; hence the coils produced descend as they 

 advance, giving rise to a curve, which is continu- 

 ally changing its plane, being converted from a 



spiral to a helix, a term of Geometry borrowed 

 from the Latin name of the common snail, which, 



