The Meramecian/Chesterian (Mississippian) boundary in the type area is currently defined as the highest occurrence of the crinoid Platycrinites penicillus and the lowest occurrence of the crinoid genera Agassizocrinus and Talarocrinus. Because these taxa have not been reported outside eastern North America, attempts have been made to use conodonts and Foraminifera to extend the Meramecian/Chesterian boundary outside the type area. Unfortunately, changes in conodont and foraminiferal assemblages do not coincide with the currently defined Meramecian/Chesterian boundary, and use of these fossil groups does not allow exact placement of the boundary outside of eastern North America.
We suggest that the Meramecian/Chesterian boundary be redefined as the boundary between Foraminiferal Zones 15 and 16i of the Mamet scheme, irrespective of the occurrence of Platycrinites penicillus. This change in definition places the Ste. Genevieve Limestone (Genevievian Stage) in the lowermost part of the Chesterian Series, with which it is biotically and sedimentologically more allied than with the underlying St. Louis Limestone. This change provides a sound biostratigraphic base for correlating the Meramecian/Chesterian boundary outside of the type area. Foraminifera, conodonts, brachiopods, and corals all show significant changes at or slightly below the St. Louis/Ste. Genevieve contact rather than at the Genevievian/Chesterian contact.