The paper gives examples of recent fresh-water molluscan assemblages from Oneida Lake, New York, Bear Lake, Utah, Potomac River at Great Falls, Maryland, and the Forest Stream of the Susaa, Denmark; and fossil assemblages from lake deposits of the Truckee and the Tulare formations, and river deposits of the Kootenai and the Fort Union formations. A correctly identified assemblage of forms from a single deposit of more or less uniform lithology is suggested as a basis for ecological interpretation. Such an assemblage should be analyzed in the light of the known ecological criteria.

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