One additional note I meant to include the other day in my post on
shell smell.  (Sandro's question on uncurling chitons reminded me of
it, even though my point has nothing to do with chitons).  Sometimes
when a mollusc is allowed to "biodegrade" instead of being promptly
removed from the shell, the operculum absorbs some of the resulting
foul-smelling fluid.  Occasionally, with a larger operc, this can be
so bad that even after the shell is thoroughly cleaned, you don't
want to display it with the operc in place.  I have had good luck
eliminating this problem by soaking the operculum in a strong
solution of liquid dish detergent (1 part detergent to 5 or 6 parts
water) for a few days, then rinsing with water and drying.  Once
completely dried, the odor is gone.
This technique is also useful for flattening opercula that have dried
in a curled position.  Have you ever tried to "unbend" an operc, only
to have it snap in half?  The detergent soaking makes them flexible
enough that they can be flattened.  Then, rinse them, place them
between two layers of absorbent material (several thicknesses of
paper towel works well), and put a brick or some other heavy, flat
weight on top.  Leave them for a few days or longer, until they are
thoroughly dry, and they will remain flat.
Paul M.