LaVerne,

I would recommend you go back to the old standby of a mixture of water and
bleach.  Make sure the murex are put in the solution spire down and maybe
swirl them around a bit to make sure you get the solution all the way into
the shell (wear gloves!!).  This will not hurt the colors of the shells.  I
have left shells in a mixture of half bleach and half water for literally
weeks (not intentionally) and no harm came to the shell (seashells only, not
land shells).  Then a strong rinse with the hose -- use a bit of pressure.
So, soak them, then lots of fresh water rinsing, and finally a bit of
silicone or mineral oil -- they'll look great (and maybe smell okay also).
If after all of this you still can't get rid of the smell, dump a spoon full
of baking soda into the aperture and plug it with cotton.  This doesn't work
with some shells as the rotting animal parts can damage the shell -- but
your murex will be okay.

Tom Eichhorst (in chilly, for this week at least, New Mexico, USA)