FORMATION OF SHELLS. 175 



The shape of the shell depends altogether on tlie extent 

 and particular form and position of the secreting organ. 

 The animal, on its exclusion from the egg, has already a 

 small portion of shell formed. The simplest case is that 

 in which this rudiment of shell is a concave disk. We 

 may conceive the animal, covered by its mantle, to expand 

 the border of this organ, and extend it beyond the edge of 

 the shell, where it then forms a new layer of shell; and this 

 new layer, being applied to the inner or concave surface of 

 the original shell, will, of course, extend a little w^ay beyond 

 its circumference. The same happens with the succeeding 

 layers, each of whichbeing larger than the one which has pre- 

 ceded it, projects in a circle beyond it; and the whole scries 

 of these conical layers, of increasing diameters, forms a com- 

 pound cone, of which the outer surface exhibits transverse 

 lines, showing the successive additions made to the shell in 

 the progress 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 proceed equally on all sides, as happens 

 in the patella. If the increase take place in front only, that 

 is, in the fore part of the mantle, the continual 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,?iv\(\ the Nautilus. INIost, 

 commonly, however, as in the Bucclniiin, and Jichatina 

 (Fig. 108) the deposite of shell takes place laterall}^, 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 changing its plane, being converted from a spiral 

 to a helix, a term of Geometry borrowed from the Latin 

 name of common snail, which, as is well known, has a shell 



calcareous covering- to the mouth of the shell. Mr. Gray also ascertained that 

 in the Cymbia, and Olivx, and the AnciUariae^ shell is deposited, and most 

 probably secreted by the upper surface of tlie foot, vvliich is very larg-e, and 

 not by the mantle, which is small, and does not extend beyond the cdg-e of 

 the mouth. 



