Thanks to all who have written on this topic. I am interested more in the
methods of rediscovery than in specific cases, because they illustrate how
anyone can rediscover a lost species (or discover a new one)--but it's
always intriguing to hear the actual firsthand story!
 
Judging from responses, species cannot be effectively rediscovered without
accurate identification. Someone actually has to recognize the specimen,
either by previous study or by realizing that it doesn't match any of the
usual species for the area. There are two lessons here: "Know your species"
and "Don't give up easily on identifications." Of course, freaks and
juveniles are not included in most identification manuals, and they may
look like new species until you're used to them. Get some help with those
last few shells that you can't identify--I mean the last few shells that
are in good shape, of course.
 
Species are quite often discovered or rediscovered in routine surveys of
river systems or other areas. There, the people conducting the survey are
generally knowledgeable about the local fauna, and have a "search image"
for supposedly extinct species.
 
It is also common for species to be "lost" because the original description
did not include a precise locality, e.g., "Amazon River" covers a lot of
territory. Collectors, like detectives, have sometimes been able to track
down the locality of a shell from all sorts of clues. For instance, "This
looks like a species that hugs the rocks, so it would be best to look for
it in the rapids."
 
I don't know if anyone has ever used this particular clue, but the sediment
remaining inside a shell can be very informative as to its origin. It may
include rock grains or microscopic skeletons of algae or foraminifera of
known range. So rediscovery can be deliberate as well as accidental.
 
It may seem trivial to emphasize the importance of accurate identification,
but consider this scenario. Suppose a collector identifies most of his or
 her shells from the Siwash River, but can't identify them all. Eventually
they end up in a museum, and years later, Tom Watters or Kurt Auffenberg
visits the museum and says, "Shazam!* Quadrula pastafazula was still living
in the Siwash River only two decades ago! I'm packing my bags and waders to
see if it's still there!" Wouldn't it have been nicer if the collector
could have known? And it certainly would have been easier for Tom or Kurt
to have a guide to the collecting site.
 
And, of course, especially nice if the collector knew it for him- or
herself, at the moment of collection. Watch out about collecting endangered
species, though. A camera might be the way to do it, plus good field notes.
 
Andrew K. Rindsberg (who rediscovered a species of Eocene chiton a couple
of years ago in a routine survey)
Geological Survey of Alabama
 
*He probably wouldn't have said "shazam".