James,
 
When you get down to the oysters and the slippers you are at the really
tough part of cleaning a shell.  Sadly, you probably will not get all of
these guys completely off and can ruin the shell if you work at some of them
too hard.  I have an apple murex I cleaned years ago and it still has the
attachment scar left by an oyster.  I soaked this shell for weeks and used
both a dental pick and in some places a small wire brush.  So, I would soak
it some more and then use a dental pick on those stubborn attached shells.
But be careful as the shell is often somewhat thin where the hitch-hiker is
attached.  In the end it is sometimes best to leave some of these alone.  I
have a small Thais almost completely covered with white barnacles.  If I
busted them off there would almost certainly be scars where each one was
attached (and they are a lot easier to get off than oysters).  Instead, I
left them all on the shell and it makes an interesting display.  If nothing
else it gives people who look at a shell collection some notion that not all
shells are perfect, bright, clean, and ready to display.  In fact, after
cleaning a couple of Murex radix a few years ago, my thoughts went way up
for anyone who cleans an encrusted murex.
 
If you hear about using acid -- don't.  The stuff is extremely dangerous and
will quickly ruin a shell; not to mention burning your lungs or eating holes
in your fingers if you slip up, etc.  Sometimes you just have to give in a
bit to mother nature.
 
Tom Eichhorst in New Mexico, USA
 
P.S.  In some of my trading I have sent Murex pomum, but of course, I keep
the one with the scar back here.  Well, I'm getting to where I think I like
it better than my "perfect" specimens.  It will be the one I have left when
all of the others are traded away.