SHRUBSOLE : EROSION OF CERTAIN FRESHWATER SHELLS, yt 



powerful action upon the substance of the shell after it has 

 pierced the epidermis, I by no means wish to convey the impres- 

 sion that I consider the case as a matter of cause and effect fully 

 proved. I would venture to suggest the lines on which the 

 correctness or otherwise of the theory may be tested. It is that 

 when shells are found eroded, that the water from which they 

 have been taken should be analysed as to the amount of lime 

 present. A series of observations of this kind would furnish us 

 with valuable data as to the per-centage of lime which is necessary 

 for moUuscan life, to successfully construct and maintain its 

 habitation. 



I am also endeavouring to confirm the view I have 

 advanced by placing a number of Planorbes from the same 

 Canal in water which I know contains at least ten grains of lime 

 to the gallon. In this case I do not expect to find the erosive 

 action will occur. These experiments will extend over some 

 months, and will form the subject of a future communication. 



It may be thought that if the theory now advanced be 

 correct, the erosion seen in Unio margaritifer ought to be more 

 general, instead of being confined to the umbonal region, and 

 seeing that there is so little lime in the water, the erosion ought 

 to be in excess of any of the other examples. The answer is to 

 be found in the remarkable thickness of the epidermis, a seem- 

 ingly needful protection for this species, living in water highly 

 charged with carbonic acid. The epidermis may be best 

 described as a thick horny coating which effectually excludes 

 all outside chemical action. 



The Planorbes have now been eighteen months under my 

 observation. Just lately I have observed an unusual modifica- 

 tion of the shell mouth. The lip ordinarily in Planorbis corneus 

 is reflected in the least degree only, but several of my specimens 

 have the aperture widely trumpet-shaped. This modification, I 

 conceive, has arisen on the part of the animal from a desire to 

 cover over as much as possible of the ravages caused by 

 erosion. In this instance we are able to assign the varietal 

 character to a physical cause. 



