The  speculation has been made several times on  the L that there may
in fact be no rare species, but only spp whose "prime habitat" has
simply not been sampled.  To a certain extent, this may be true, but in
nature there is  actually much data to suport the notion that some,
perhaps many species are indeed truly rare, either in the sense that
they exist in very low population densities, even in what appears to be
their breeding habitat, or in the sense that their preferred habitat
itself, is extremely scarce.  This may be especially true for Turridae -
there are several spp of Turrids in my region which are known from a
very few specimens, even though there have been tens of thousands of
samples taken around where the specimens have been found, using a
variety of sampling methods.  It could be argued that these spp might
just be very fast, but this is not convincing.
        Many spp in all phyla have been known to exist and perpetuate
themselves at very low densities - consider for example species or
subspecies nearly extinct for one reason or the other: Siberian Tigers,
many species of whales, some brachiopods, ...some driven to the edge of
extinction by man, others just at the end of their time on the planet.
Other species perhaps have extremely efficient methods of finding mates
at the proper time (such as powerful phemerones, or visual clues visible
for  signifigant distances), so they do not have to have high densities
in order to survive.
        So, i think it safe to say that yes, there are indeed rare species in
fact, as well as in collections.
-Ross M.
--
Ross Mayhew:    Schooner Specimen Shells:    Http://www.schnr-specimen-shells.com
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