You could start with A. Myra Keen & John C. Pearson "Illustrated Key to
West North American Gastropod Genera" Stanford U. Press, 1952 39pp.
and A. Myra Keen & Don L. Frizzell "Illustrated Key to West North American
Pelecypod Genera" 32 pp. 1953
John Wolff
2640 Breezewood Dr.
Lancaster, PA 17601, U.S.A.