The Hell Creek Formation of eastern Montana, which forms the top of the Cretaceous section, contains in addition to a rich vertebrate fauna a distinctive assemblage of pelecypods, made up of three species of Plesielliptio, one of Rhabdotophorus, two of Plethobasus, one of Quadrula, nine of Proparreysia, one of Obovaria?, one of Corbicula, and one of Sphaerium. Many of the species show distinctive ornamentation, analogous to that of living Unionacea in the Mississippi drainage. One species of Plesielliptio, one of Proparreysia, and one of Sphaerium are new. There is some community of species with the faunas of the Lance and Black Buttes formations of Wyoming, less with the older Cretaceous assemblages, and none with the abundant mollusks of the Paleocene Fort Union formations.

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