>Like most molluscan shells and almost all gastropods (apart only from Epitonioidea), Crepidulidae are composed largely of aragonite. There will be some calcite layers, but very thin.<

A clarification: Some other gastropods and other mollusks have a thin calcite layer.  Most gastropods and bivalves, including Crepidula, are entirely aragonite.  Naticids, some limpets, and some muricoideans have an outer layer or partial layer of calcite.  Eupteriomorph bivalves such as pearl oysters, oysters, scallops, anomiids, and limids have an outer layer of calcite and sometimes calcite inner layers as well.  Various extinct mollusks also had calcite in their shells.  Major muscle attachment areas are always aragonite, even when the rest of the shell is calcite.

>Mussels, for interest (Mytilidae) are largely composed of calcite.<

Mytilus itself is largely calcite, but with some aragonite as well.  Other genera in Mytilidae range from entirely aragonite to largely calcite.

Calcite and aragonite are two different crystal forms of calcium carbonate, just as diamond and graphite are two different crystal forms of carbon.

    Dr. David Campbell
    Old Seashells
    University of Alabama
    Biodiversity & Systematics
    Dept. Biological Sciences
    Box 870345
    Tuscaloosa, AL  35487 USA
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That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa