FORMATION OF SHELLS. 241 



ment, and the subjacent cerium, which is incor- 

 porated with the mantle, and may be regarded 

 as forming one and the same organ.* 



The shape of the shell depends altogether on 

 the extent and particular form and position of 

 the secreting organ. The animal, on its exclu- 

 sion 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 sur- 

 face of the original shell, will, of course, extend 

 a little way beyond its circumference. The 

 same happens with the succeeding layers, each 

 of which being larger than the one which has 

 preceded it, projects in a circle beyond it ; and 

 the whole series of these conical layers, of in- 

 creasing diameters, forms a compound cone, of 

 which the outer surface exhibits transverse lines, 

 showing the successive additions made to the 



* A secreting power is also, in some instances possessed by 

 the foot, as is exemplified in some of the gasteropoda, where it 

 forms an operculum, or calcareous covering to the mouth of the 

 shell. Mr. Gray also ascertained that in the Cymhia, the Olivce, 

 and the Ancillarice, shell is deposited, and most probably se- 

 creted by the upper surface of the foot, which is very large, and 

 not by the mantle, which is small, and does not extend beyond 

 the edge of the mouth. 



VOL. I. R 



