What is a fossil bivalve mollusk of the genus Hippurites, of many species, having a conical, cup-shaped under valve, with a flattish upper valve or lid. Hippurites are found only in the Cretaceous rocks called?

hippurite — A fossil bivalve mollusk of the genus Hippurites, of many species, having a conical, cup-shaped under valve, with a flattish upper valve or lid. Hippurites are found only in the Cretaceous rocks. (Webster's 1913.)

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