Notes on the effects of the atmosphere on the shells of mollusca

I think it is well for the benefit of conchologists to put down some observations on this point. I have frequently noticed the fading of colour so observable in some specimens.
I have some Cyprea which some years ago were found alive at Portland, Vic, crawling over the rocks there, at one time so abundant. Such specimens as I still have are now of a light grey colour, which when first procured were so dark as to be almost black. This I had attributed, along with most others with whom I had any conversation on the subject, to the effects of the sunlight ; however, this in my experience has proved to be an error, as it is the results are evidently due to the atmosphere and not the light, as heretofore supposed.