Ross Mayhew has a good point: we are still in the Golden Age of discovering
new species of mollusks.
 
The way this works, historically, is that collectors found the most
conspicuous stuff first--starting with the biggest and brightest shells on
the shore--and it got progressively more difficult from there. In the 19th
century, the big finds were on shorelines, reefs, continental shelves,
rivers, and on land--although not necessarily in the most accessible
regions, because people have always transported shells as art objects. Now,
people are finding new shells in the deep sea, in caves within reefs, and
under the microscope.
 
The same thing happened with fossil shells in the U.S. Coastal Plain.
First, the shells that were tough enough to weather out of the rock and lie
on the ground just waiting to be picked up (e.g., oysters and scallops,
both made of calcite). Then, the shells that had to be dug out, but were
common (e.g., aragonitic shells). It is still not too difficult to discover
new species even at the best-known sites, but generally of rare or fragile
or tiny mollusks, sometimes, indeed, of mollusks that are simply hard to
tell apart--even the occasional oyster or scallop.
 
Andrew K. Rindsberg
Geological Survey of Alabama