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Oysters, whose inner shell layer contains chambers, vesicles, and sometimes chalky deposits, often have extraordinarily thick shells of large size, prompting the idea that there is something unusual about the process of shell fPormation in these and similarly structured bivalves with the oyster syndrome. I propose the hypothesis that calcifying microbes, especially sulfate-reducing bacteria growing on organic substrates in fluid-filled shell-wall chambers, are responsible for shell calcification away from the shell-secreting mantle of the host bivalve. Other phenomena, including the formation of cameral deposits in fossil cephalopods, the cementation of molluscs and barnacles to hard substrata, the formation of a calcified intriticalx on the shell's exterior, and cementation of objects by gastropods on the shell for camouflage, may also involve calcifying bacteria. Several lines of inquiry are suggested to test these hypotheses.