Ross,
 
I believe the "black ovulid/cypraeid thing" is Chimaeria incomparabilis
and I believe there was one price of $20,000 for it but I seem to
remember prices of $50,000 or so being talked about.  Another seldom
seen shell is Cymatium armatum, know only from a broken shell for
decades.  Personally, when a shell starts hitting the $100 range, I
think of it as rare, and when it hits the $1,000 range like the recent
Cypraea leucodon on the Raines auction (great auction, Bret!!), I think
of it as not available -- a perfect candidate for photographic
representation in my collection.
 
I know, I know, all of you purists out there who hate it when I talk of
shell values are saying that high price does not equal scarcity or
rarity.  True enough but there is enough of a correlation for my
purposes.  If we are talking self collecting, then they are all very
rare from my perspective as I can walk the beaches of the Rio Grande del
Norte for eons and only find the occasional pond snail and the
introduced Asian clam (Corbicula fluminea), plus some other stuff you do
not want to hear about.
 
So it comes down to a question of the definition of rarity.  Are we
talking about a shell seldom seen but that may be plentiful in its
natural habitat -- for instance almost all microshells?  How about a
shell that is hardly ever seen in collections -- Berthelinia sp?  Or is
it a shell that has only a few specimens representing the species -- no
example here because we just don't know.  At one time it was Conus
gloriamaris or Strombus listeri -- today it must be Chaemeria
incomparabilis (at least until some trawler pulls up a few hundred).
 
Maybe the truly rare shell is the one we will keep no matter what.  The
one with a story behind it -- the first collected, a gift from Grandma,
a gift from a friend, etc.  I don't have a first collected shell or one
from Grandma -- but I have some from different friends that are rare
treasures.  These are the ones that evoke an image or memory.  When I
look at a certain land snail I think of Dan, a Latiaxis of Lynn, a
volute of Bill, another land snail of John, a long sought after cassis
of Carlos, etc.  Like most of you, these are my valuable shells and in
my commercial way of thinking, the rarest.
 
And this is what happens when you mix up Pina Coladas and your wife only
has half a glass, leaving the rest to you.
 
Tom Eichhorst in New Mexico, USA