< The speculation has been made that it is safe to say there are rare
shells, yet no scientific evidence that I am aware of, was offered to
support this statement. As such it becomes an opinion, no better or
worse than those offered by others, but just an opinion.>: this quote is
selected because it is the most concise utterance of one line of thought
on this topic.
        Several possible reasons for species being "truly rare" were put forth
: 1) the "evolutionarily declining" species Hans Turner wrote about
(which if evolution is truly the way species develop, most probably
exist, perhaps in large numbers!),  ( some species  have developed
methods for breeding even though they may have extremely low densities,
so spp "at the end ot their time" could hold out for protracted periods
in some cases!!) 2) species which live only in extremely uncommon
habitats, 3) spp which have a very limited geographical distribution  ,
and 4) species which are becomming endangered by non-evolutionary
influences (such as human) - i am sure several other categories can be
postulated also.
        It is an established fact that a good number of species are known from
only a handful of specimens, often despite heavy sampling by various
methods in the areas where they are found.   While this does not
nessessarily mean ALL such spp are in fact "truly rare", as many of the
species falling into the above categories may be expected  to be, taken
as a global fact it is a strong indicator that there are indeed species
which are just not at all common in nature, independant of their
representation in collections, scientific or private.
          On a strictly common-sense note, it seems likely that total,
planetary populations of species fall along some sort of distributional
curve, with abundant spp such as Cypraea moneta on one end, and extreme
examples of the type of categories mentioned above being more likely to
fall towards the other ("numerically low global population") end of the
curve.  **By definition**, the species on the extreme ends of such a
distibutional curve could be designated "abundant" and "rare".  It must
be noted, however, that it is seldom possible to identify which species
probably belong to the "extremely rare" category (based upon this
particular difinition of rarity, ie - other operational definitions
could include spatial distribution, range, the total number of local
populations, etc.), given our often limited information-base at present.
  What is certain, however, is that no matter HOW one defines rarity, a
certain percentage of species WILL fall into this category, simply "by
definition".  The "rarer" a species actually is, the more data will be
required to determine its actual status, hence the more difficult it
becomes to make an accurate designation, ie. to attatch numerical
estimates with acceptable statistical confidence levels.  Thus, no
matter how defined operationally, some species will truly be "rare" and
"extremely rare", but determining which spp actually fall into these
categories is often quite difficult, given the resources typically
avaliable for this type of enquiry.
-Ross M.