Nora,
  For the eastern part of the United States I frequently refer to Emerson
and Jacobson, The American Museum of Natural History Guide to Shells,
copyright 1976.  It covers marine, freshwater and terrestrial mollusks
from Maine to Florida from the Appalachian Mountains eastward to the
coastal area.  The book has some drawbacks, mostly b&w illustrations, a
few in color; also the determination keys get some taking use to.  I
believe this book is out of print, however, I occasionally see it in the
catalogs of dealers of used natural history books.

Charlie
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Charlie Sturm, Jr
Research Associate - Section of Invertebrate Zoology
                     Carnegie Museum of Natural History, Pittsburgh, PA
Assistant Professor - Family Medicine

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