Art,
I think that question may be impossible to answer, because it is very
difficult, if not impossible, to tell if a marine species IS extinct.  What
criterion could be used?  No specimens collected in the past 50 years?  What
would that prove, considering that less than 1% of the ocean floor has been
explored in the past 50 years?  Slit shells were "known" for many years to be
extinct as a family, until a live one was brought up in a net.  Not to
mention the coelacanth!  Actually, it is impossible even to know with
certainty whether a marine species is truly "rare", because we have explored
so little of the ocean floor - as shown by the recent glut of Cypraea fultoni
on the market - a species that was "extremely rare" just last year!  At the
recent COA convention, they were all over the place!  It is much easier to
determine if a land snail has been exterminated from the one island it
occupied, or a fresh water mussel from the one river it lived in, than to
know if a species has been exterminated from the ocean!  Also, it is far less
likely to happen in the ocean.
Paul M.