For those of you who do not have access to a copy of "A Listing of Living Mollusca" by Goto and Poppe, published in 1996, I thought that some comments from the introduction concerning numbers would be of interest.
 
The introductory page states:
 
"The estimates of the number of living species (Solem 1974, 1984 and our present studies) are as follows:
 
Gastropoda      land    33,000
        freshwater      3,000
        marine  45,000
Bivalvia        freshwater      1,500
        marine  14,000
Cephalopoda             1,000
Polyplacophora          1,000
Scaphopoda              350
Aplacophora             300
Monoplacophora          20
 
This is a conservative estimate which totals nearly 100,000 different species.
 
About 300,000 names have been proposed in malacology (including subspecies and forms).  Peter Dance (1986) and Solem (1974) highlight this activity:
        1758  Linnaeus described about  700 species
        1817  Dillwyn lists     2,244 species
        1850  H. & A. Adams estimate    17,321 species
        1891  Paetel lists      44,482 species
 
At the time of Paetel, the number of published papers on mollusks was somewhere between 9,000 and 15,000.  According to Solem, over 150,000 papers have been published now.
 
The present work lists 41,861 species and 4,409 subspecies."
 
The authors conclude: "that this work is only a beginning, and that future editions may lead to a more complete listing of the living Mollusca, arranged according to one classification, and approaching the reality of what exists in nature.  For this reason, we also invite authors who want to have their books referred to in future editions to make contact with our offices".
 
For further details, please contact one of the authors, not me!  I hope they do not object to my copying out the above without their permission.
 
Tom Walker