Conventional wisdom, based on properties of reagent-grade salts, states that organisms which construct their exoskeletons from calcite rather than aragonite have a greater ability to resist dissolution. However, experiments on individual bivalve microstructures show that other factors such as crystal size and the proportion of organic matrix outweigh constituent mineralogy in determining the rate of shell loss in cold waters. Moreover, the loss of shell material was minimal, corresponding to an annual loss in shell thickness of 3–40 μm depending on microstructure. These findings must cast doubt on the commonly held supposition that calcitic layers added to the outside of shells are an adaptation against dissolution in Holocene ‘Aragonite’ seawater.