The basis for the comparison of American with European lower Tertiary mollusks has been the following: For America, the collections in the United States National Museum and the monographs of G. D. Harris1 on the Midway and “Lignitic” (Wilcox group) mollusks and on the “Pelecypoda of the Saint Maurice and Claiborne stages.” For Europe, the works of Briart and Cornet2 and of Cossmann3 on the Montian; of Cossmann and Pissarro4 on the Eocene of the Paris Basin; of Oppenheim5 on the Priabonian of Italy; of von Koenen6 on the lower and middle Oligocene of North Germany; of Speyer7 on the upper Oligocene of North Germany; and the rather meager collections of European fossils in the United States National Museum. Few direct comparisons of shell with shell have been possible; more often shells have been matched with figures, or simply plate with plate.
The mollusks of the Midway group show little in common with any European stage. As the Midway is the lowest group of the eastern American Eocene, one would naturally look for affinities in the Montian of Europe. . . .