Jorge,
  I might suggest the following works for freshwater and terrestrial
material
 
R. Tucker Abbott, Compendium of Landshells, copyright 1989
Brian Parkinson, Jens Hemmen and Klaus Groh, Tropical Landshells of the
World, copyright 1987
These are two works that treat shells found around the world.  There are
also many works of a more regional level.
 
For freshwater mussels=Unionids=Naiads I might suggest you look at the
following work:
Shohei Shirai, Pearls and Pearl Oysters of the World, copright 1994.
  The illustrations in this work are not the best, however, it is the only
work that covers the unionids worldwide.  This book also covers the
marine bivalves and gastropods that can form pearls.
 
  I do not know of any work that provides worldwide coverage of the
freshwater gastropods.
 
You might also want to check out the Smithsonian Institutions web site for
other recommendations.
http://www.si.edu/resource/faq/nmnh/mollusk1.htm
 
Good luck
 
Charlie
******************************************************************************
Charlie Sturm, Jr
Research Associate - Section of Invertebrate Zoology
                     Carnegie Museum of Natural History, Pittsburgh, PA
Assistant Professor - Family Medicine
 
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