III.] INTRODUCTION. li 



Mollusca, as well bivalve as univalve^ are frequently ex- 

 coriated or decorticated, and sometimes to a considerable 

 depth when the shell is thick, as in Unio margaritifer, 

 or the pearl-mussel. Several marine species, especially 

 those oiAstarte, Mytilus, and Littorina, are also affected 

 in the same way. Various theories have been put for- 

 ward to account for this erosion. In the case of fresh- 

 water shells, many naturalists have supposed that it is 

 caused by gaseous action, some have attributed it to the 

 attacks of Myriapodous insects, a few to excavation by 

 the eggs of Neritina fluviatilis, MM. de Saulcy and Fis- 

 cher, as well as Mr. Bland, to other Mollusca eating away 

 the calcareous matter for the purpose of constructing 

 and repairing their own shells, and MM. Cuigneau and 

 Lespinasse to parasitic Confervse and other hydrophytes. 

 But as this phenomenon is not confined to freshwater 

 shells, some other explanation must be sought for. It 

 is very probable that the former class of cases may be 

 attributable to chemical action, and especially that of 

 sulphuretted hydrogen ; but in the other, and perhaps 

 all the cases, the effect may be produced by another 

 cause. Mr. Grove suggests that it may be owing to the 

 want of homogeneity in the substance of the shell, and 

 that the slightest and almost imperceptible inequality in 

 the surface would result in an electrolytic action of the 

 water, which would gradually dissolve portions of the 

 shell. Sea-water is more likely to produce this effect 

 than fresh water, by reason of the stronger galvanic pro- 

 perty of the former. Corrosion of metals by water or 

 moisture may be due to the same cause. As the sub- 

 ject does not appear to have received much attention in 

 a chemical point of view, I trust the following remarks, 

 from so great an authority as Mr. Grove, wiU help to 

 throw some light upon it. The shells which I submitted 



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