It could be hard to rule out the possibility of something managing to attach itself while a shell was alive, even on smooth-shelled taxa that presumably have the mantle covering the shell much of the time.  I have a fossil Aurinia with medium-sized barnacles attached to the dorsal side.  The barnacles are largely covered by additional shell deposits, so they obviously settled while the snail was alive.  However, Aurinia is not quite so glossy as a Cypraea, and the photos of live volutes usually have more shell exposed than do live cypraeids.

    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