216 THE MECHANICAL FUNCTIONS. 



edge of the shell, where it then forms a new layer 

 of shell ; which new layer is parti}' applied to the 

 inner or concave surface of the original shell, and 

 partly extends a little way beyond its circum- 

 ference. The same happens with the succeeding 

 layers, each of which, being broader than the pre- 

 ceding, projects in a circle beyond it ; and the 

 whole series of these conical layers, of increasing 

 diameters, forms a compound cone, of which the 

 outer surface exhibits transverse lines, showing the 

 successive additions made to the shell in the pro- 

 gress of its increase. The Patella, 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, as in 

 the preceding instance, proceed equally on all 

 sides. If the increase take place in front only, 

 that is, in the fore part of the mantle, the continual 

 deflection thence arising necessarily gives the shell 

 a spiral form, the coils of which are all in the same 

 plane ; constituting what is called a discoid shell. 

 This is the case in the Planorbis (Fig. 105, p. 

 204), the Spirula, and the Nautilus. Most com- 

 monly, however, as in the Buccinum, and Ackatiiia 

 (Fig. 108), the deposit of shell takes place late- 

 rally, and more on one side than on the other ; 

 hence the coils produced descend as they advance, 

 giving rise to a curve, which is continually chang- 

 ing its plane, being converted from a spiral into a 

 helix, a term of Geometry borrowed from the Latin 

 name of the common snail, which, as is well known, 

 has a shell of this form. These are called turbi- 

 uated shells. Fig. 108, which represents the shell 



