Of course some shells are less common than others.  We can call the species
that are very uncommon "rare."   This same phenomenon has been noted for
every group of organisms.  Some species are common and abundant, some
are less so, and some are very much less so.  Some are widespread and some
are very localized in their distribution.  It would be foolish to assume that
marine mollusks deviate from this general pattern of nature simply because we
cannot adequately sample some ocean habitats and therefor do not have
conclusive evidence of their rarity.  If a net suddenly comes up from the depths
with 100 specimens of a species that previously had been seldom seen, we
cannot conclude that they are abundant.  Perhaps so, or perhaps they were the
last 100 alive on the earth.  This can happen.  When William Bartram collected
Franklinia alatamaha in Georgia in 1765, he apparently found the only grove of
these trees on earth.  A few years later, he collected seeds and saplings from
this same tiny population and so luckily this species still survives in cultivation,
for it has never again been found in its native habitat, even at the original
locality.
 
kate
 
kate