A recent study on opisthobranchs suggested that Boss's estimated worldwide total underestimates the Indo-Pacific opisthobranch fauna alone and thus is rather short of the global total.
Pierre Bouchet has been trying to keep up with all proposed names and some estimation of how many of the proposed names are any good.  I do not remember recent totals from that work.
The rate of discovery of new taxa certainly suggests that generous estimates may be more accurate.

    Dr. David Campbell
    Old Seashells
    University of Alabama
    Biodiversity & Systematics
    Dept. Biological Sciences
    Box 870345
    Tuscaloosa, AL  35487 USA
    [log in to unmask]

That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa

---------- Original Message ----------------------------------
>From:         "Kevin S. Cummings" <[log in to unmask]>
>Reply-To:     Conchologists of America List <[log in to unmask]>
>Date:          Wed, 24 Jul 2002 15:20:09 -0500

>According to:  Boss, K.J.  1970.  How many species of mollusks are
>there? (Abstract).  American Malacological Union, Inc. Annual
>Reports.  1970(37):41.
>
>47,000
>
>Kevin
>--
>Kevin S. Cummings
>Illinois Natural History Survey
>607 E. Peabody Drive
>Champaign, IL 61820
>[log in to unmask]
>http://www.inhs.uiuc.edu/cbd/collections/mollusk.html
>
>Join the Freshwater Mollusk Conservation Society Today!
>http://ellipse.inhs.uiuc.edu/fmcs/
>