Best way to clean Cypraea in my experience, is the freeze thaw, freeze thaw,
flush method.

Freeze shells in plastic bag. Leave outside in the sun to thaw for a couple
of days. Freeze again, leave out for a few more days and then the water pick
of the jet nozzle of a hose. Works every time. For smaller Cypraea just one
freezing is enough. We've done thousands of shells this way.

Frank (getting ready for the turkey, in Massachusetts)

Gems From The Sea
www.shellgems.com