I tend to be on the "optomistic" side of this debate- the molluscan
fauna in many parts of the world is incredibly poorly known, exp. the
tiny ones, and those inhabiting deeper-water habitats difficult to
dredge.  In a past incarnation of this debate, i told of how 2
determined collectors found about 1200 spp of xhell-bearing molluscs on
the southern shore of Grand Bahama Is., with easily 300 of them being
undescribed (mostly micros).  This was in shallow water, at a previously
well-examined  locality, and they were not that interested in bivalves,
so the 300 is only part of the surprise-package!!   Also, a recent
expedition to Indonesia yielded a bumper crop of new spp., mainly by
dredging, which misses a lot of habitats.  The deep sea is another side
of the dice- it may contain several times as many spp as occur in
shallow water, since it is so vast and diverse, and so little explored.
So, i would not at all be surprised if when all the dust settles in a
few hundred years, a half million species of molluscs are found to be
inhabiting our little planet (half of them tiny Turridae!!).
                                                                                                                                                                                -Ross M.