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into this body of water after every shower, a considei-able quan- 

 tity of saline alkalis finds its way into the water, where, in con- 

 sequence of its specific gravity, it falls to the bottom, and every 

 shell within reach of the influence of this alkaline matter, is more 

 or less eroded, and most of them very much so. But further 

 down, the shells grow more perfect, probably in consequence of 

 the dilution of the alkalis, and their more general diffusion in the 

 whole body of the water, by the influence of the slight current 

 in it. 



It may be thought strange that the presence of saline alkalis 

 in water is urged as a cause of the erosion of shells, but it may 

 be explained in this way. Wliere two or more alkalis are present 

 in the food of an animal, and only one of them is necessary and 

 proper to enable it to perform its healthy functions, the others 

 may, in part, take the place of the proper substance, and if so, 

 the shell formed under such circumstances w-ould be more or less 

 liable to erosion, in proportion to the solubility of the substituted 

 materials. 



We have now only to inquire respecting a locality producing 

 eroded shells, — Is the water so highly charged with lime, that the 

 presence of a more soluble alkali in small quantity can have no 

 material influence in the formation of the shells ? If the answer 

 be yes, then we may reasonably ascribe the eroded character of 

 the shells of such a locality entirely to minute parasites ; but if 

 there be a preponderance of saline alkalis in the water, they may 

 be reasonably expected to enter into the organization of the shells, 

 and a very slight abrasion of the epidermis of the shell from any 

 cause, would expose the soluble alkalis to the solvent action ot 

 water alone, and the remaining portion of the shell becoming less 

 dense (and '"chalky") by a removal.pf a portion of its substance, 

 would, of course, wear away very rapidly. It is easy to understand 

 why the beaks of bivalves, and the apices of univalves are first 

 attacked by the erosive process. Firstly, the epidermis is thin- 

 ner at those points : secondly, those portions of the shell formed in 

 early life may be presumed to contain more gelatinous, and less 

 calcareous matter, than the parts formed at or near maturity. I 

 do not know demonstratively that this is the case, but analogy 

 teaches it. 



