Hi Sue,

Welcome aboard the good ship Conchology.  Yes, we do discuss shells here
sometimes.
There are a great many good websites relating to shells and shelling, and
many of them have a lot of good pictures of shells, but few sites offer as
systematic or complete a pictorial presentation as books do.  Frankly, in my
experience, trying to identify a shell by surfing the net can be an arduous
process, with limited return for the time and effort invested.  You would be
better off buying or borrowing or checking out from the library a few good
comprehensive books, either thoroughly covering a particular family of
mollusks, or thoroughly covering the mollusks of a specific geographic area.

Some good "starter" websites are:
http://coa.acnatsci.org/conchnet
http://www.seashell-collector.com
http://www.molluscs.net
http://www.manand mollusc.net

I don't mention these particular sites because they are necessarily the
"best" sites, but because they each provide many links to other sites, so
they are a good "jumping off point" to find the sites you may find most
useful.

I can almost guarantee that none of your recent (non-fossil) shells are
extinct (with the possible exception of a few land snails, if you have any).
It is virtually impossible to declare a recent marine species extinct,
simply because there is so much unexplored habitat in the ocean, where any
given species might still exist.  Slit shells (family Pleurotomariidae) were
long "known" to be extinct, until a living specimen was taken in deep water
about fifty years ago (more or less - I'm not certain about the date).  Now
there are some 20 or so living species known.  To say that a marine species
is "scarce" or "uncommon" or "rare" really just describes the frequency of
that species in collections, not necessarily in nature.  More often than
not, a species considered "rare" will eventually be upgraded to "uncommon"
as more specimens are found, or even to "common", once the principal
breeding populations are discovered.  Tucker Abbott, in his classic book
"Kingdom of the Seashell" said (as nearly as I can remember) that for any
species living today there must exist a breeding population of at least
several thousand individuals in order to support the existence of that
species.  The relative "rarity" of a species therefore is usually a matter
of whether or not we have yet discovered where it lives.  Land snails are a
somewhat different matter, since some species are restricted to a single
island, or a small group of islands.  If such a species has not been
collected in fifty years or so, and the species has never been found in any
other geographic locality, then it might reasonsbly be declared extinct.
But of course such a declaration, like everything in science, necessarily
remains subject to change in the light of new evidence.

Paul M.