In speaking of "rare shells" one has to differentiate between "rare in
collections" and "rare in nature".  For terrestrial mammals and birds,
generally speaking, rare is rare, and rarity in collections corresponds to
rarity in nature.  We know for a fact that giant pandas and mountain
gorillas and California condors are rare in nature.  No-one is going to
discover a valley containing thousands of individuals of these species.  How
do we know?  Because most of the possible habitat for these species has
already been explored.  With marine species however, the opposite is true.
Even though a species might be known from only one specimen, it is highly
likely that there is an undiscovered area where thousands of specimens
exist.  Why?  Because less than one percent of the ocean floor has been
explored to date.  Slit shells were once "known" to be not just rare, but
extinct!  Now we know that they are not extinct, and in fact are quite
common in some of the places they live.  Still, they remain uncommon to rare
in collections because of the inaccessibility of their habitat.  A couple of
years ago Cypraea fultoni was a "rare shell" - meaning, it was rare in
collections.  But it only took one trawler operating in the right locality
to demonstrate that the species is not rare in nature at all.  It was only
rare in the areas where we had previously collected.  But it is quite
abundant in other locales.  And now, inevitably, it is becoming less rare in
collections.  R. Tucker Abbott summed it up nicely in his discussion of rare
shells in his "Kingdom of the Seashell" (this is a paraphrase since I don't
have the book in front of me) - "There may well be no such thing as a rare
marine species.  Any species living today must have a population large
enough, surely of several thousands of individuals, to ensure the ongoing
propagation of the species".
Paul M.