uppermost rocks of the Ordovician (Ordovician Period) System in North America, famous for their fossils. This series is defined on the basis of rock exposures in the vicinity of Cincinnati, Ohio, including southwestern Ohio, northern Kentucky, and southeastern Indiana. The rocks of the Cincinnatian Series were deposited between approximately 451 and 443 million years ago.

In its type area the Cincinnatian Series consists of approximately 300 metres (1,000 feet) of highly fossiliferous limestones (limestone) and shales (shale). Fossils, which are extremely abundant and well preserved in this series, are readily found in almost any exposure of these rocks along a roadside or in a stream. The most common fossils include brachiopods (lamp shells) (lamp shells), bryozoans (moss animal) (moss animals), bivalves (bivalve), gastropods (gastropod), and cephalopods (cephalopod). Although rarer, complete trilobites (trilobite), crinoids (crinoid), and edrioasteroids (starfishes (sea star)) are routinely found by both professionals and amateur collectors.

Many professional paleontologists (paleontology)—including E.O. Ulrich, an influential stratigrapher and former head of the U.S. Geological Survey, and Charles Schuchert (Schuchert, Charles), a leader in the development of paleogeography—started as amateur fossil collectors in the area surrounding Cincinnati, Ohio.