resembling modern species found in those strata. He used Greek names: Eocene, Miocene, Older Pliocene, and Newer Pliocene.

Although these divisions seemed adequate for the region to which the designations were originally applied (parts of the Alps and plains of Italy), when the same system was later extended to other parts of Europe and to America, it proved to be inapplicable. Therefore, the use of mollusks was abandoned from the definition