IX 



DEPOSITION AND FORMATION OF SHELL 



255 



Formation of Shell. ^ — The mantle margin is the principal 

 agent in the deposition of shell. It is true that if the shell be 

 fractured at any point, the hole will be repaired, thus showing 

 that every part of the mantle is furnished with shell-depositing 

 cells, but such new deposits are devoid of colour and of periostra- 

 cum, and no observation seems to have been made with regard 

 to the layers of which they are composed. As a rule the mantle, 

 except at its margin, only serves to thicken the innermost layer 

 of shell. 



It is probable that the carbonate of lime, of which the shell 

 is mainly composed, is separated from the blood by the epithelial 

 cells of the mantle margin, and takes the cr3'stalline or granular 

 form as it hardens on exposure after deposition. The three 

 layers of a porcellanous shell are deposited successively, and 





Fig. 163. — Sections of shells. A. Conus: a, outer layer: h, middle prismatic layer, 

 with obliquely intersecting laminae above and below ; c, inner layer. B, Oliva : 

 a, outer layer; b, layer of crossed and curved laminae; c, prismatic layer, suc- 

 ceeded by lajer of laminae at right angles to one another; d, inner layer. C, 

 Cypraea: a, outer layer; 6, middle layer; c, inner layer. 



the extreme edge of the mouth, when shell is forming, will 

 contain only one layer, the outermost; a little further in, two 

 layers appear, and further still, three. The pigment cells which 

 colour the surface are situated at the front edge of the mantle 

 margin. 



Shelly matter is deposited, and probabl}^ secreted, not only by 

 the mantle, but also in some genera by the foot. This is cer- 

 tainly the case in Cymhium^ Oliva^ Ancillaria., Cassis^ Distortion 

 and others, in several of which the foot is so large that the 

 shell appears to be quite immersed in it.^ 



1 See also p. 258. 2 j, e. Gray, Phil. Trans. 1833, p. 774 f. 



