Dear Travis:
   Now this is moderately serious: I think any estimate of numbers
should be broken down a bit. I think there would be several
catacories: 1. Marine Bivalves, 2. Marine gastropods, 3. Marine
"other", 4. FW bivalves. 5. FW gastropods, 6. Land snails.
  This way, experts in each of the six fields can contribute their
estimates. It would indeed be interesting to see those totals.
          Art