Looks like we are coming to a consensus: "rarity" is simply  a matter of
definition, ie labelling. A dealer's or  private collector's definition
of a "rare" shell can  be quite different (and far less precise!!) from
a scientist's definition, and i muddied the water initially by placing
scientific and commercial aspects of the issue in the same message!! The
world of private collections and buying and selling of shells will
consider something rare if it is hard to obtain given the resources
available to that community, and this designation is only tangenitally
related to the scientific definition and designation of rarity.  We have
a problem with limited sampling data for many species, so the
confirmation that any given taxon is "rare" in the absolute, scientific
sense, is often difficult or in the case of deep-ocean species,
well-nigh impossible.  That said, by ANY definition of rarity, there are
in fact truly rare species and forms: this much *cannot* be disputed,
since it is only a matter of semantics and statistics: it is the
*confirmation* of any tentative designations of rarity which causes
problems.  Yet, even here, one can confirm relative abundance and
distribution for shallow-water species in well-sampled areas: for
example i can say with some confidence that three normally arctic spp of
Turridae are rare in Nova Scotian waters less than 200m deep, since
areas delimited by these parameters are in fact reasonably well sampled,
and these three species occur only in one small area (which i suspect to
be a remnant population of ice-age fauna, incidentally) - so for these
three at least, i can say they are *truly* rare in near-shore waters in
the Nova Scotian region. (however, i cannot say if they are rare
globally, because of extremely limited sampling data from other possible
habitats and regions where they may occur in high densities.)
Verbosely yours, with very sore elbows,
Ross M.