Hi Barbara,
Unfortunately, the shells with the broken down foam rubber on them are
probably badly damaged.  The breakdown of such products releases acids
which damage the shells.  I once bought an old collection which included
many small plastic boxes with foam inserts.  Many of them had been
stored in a damp cellar, for many years, and the foam was just as you
described - a hard, gooey mass adherent to the undersides of the shells.
 Cowries had not only lost their gloss, but even their smoothness.  The
bottoms of the shells, once cleaned, actually looked and felt rough.
Some of the thinner shells, especially the wentletraps, actually had
their undersides eaten away, so there was essentially just half a shell
left, or at least several large holes in the underside.
As far as removing the foam residue, bleach won't touch it.  Try
scrubbing it off with a stiff brush, using warm water and plenty of
detergent.  If that doesn't do it completely, you'll need some kind of
organic solvent.  You could try mineral spirits, paint thinner, charcoal
lighter, etc.  These won't hurt the shells.  Hopefully some of the
shells will be worth saving, though I'm not too optimistic.
Regards,
Paul M.