Hi Carol,
When I took undergrad invertebrate zoology, most of the textbooks
stated that there were "somewhat over 80,000 described species of
Mollusca".  However, that was shortly after mollusks were discovered.
 Most of what I have read recently puts the number somewhere between
100,000 and 120,000 described species.  Of course, not all of them
have shells, but most do.  The same old textbook says there are an
additional 35,000 described fossil species.  I wouldn't be surprised
if that number has increased by a factor greater than that of the
living species.  The mollusks are the second largest phylum of
animals, exceeded in numbers only by the Arthropods.  Of course, the
Arthropods get top billing largely thanks to the inclusion of that
one mega-class of animals we all know and love so well - the insects.
Paul M.