The early part of the Carboniferous Period of the Paleozoic Era, from about 360 to 310 million years ago, is given the name "Mississippian" in North America. European geologists do not make this distinction.
During the Mississippian period in Laurussia, proto-Appalachian Mountains formed by the Acadian Orogeny (during the Devonian Period) were busy eroding away into a peneplain. A shallow tropical sea to the west of the mountains laid down the massive limestone, sandstone, and shale layers that dominate the Midwest (and lie under the Gulf Coastal Plain rocks of Eastern Mississippi). On the eastern side of the mountains, sediments washed onto the continental shelf formed limestones and sandstones that now lie under most of England and parts of Wales. The predominant Mississipian fossils are crinoids. Rocks that would later form the Ouachita Mountains and the Ozark Plateau were laid down.
By about 310 million years ago, the sea to the west of the proto-Appalachians had mostly filled in. The changes wrought by this would result in different rocks being laid down in the later half of the Carboniferous, the "Pennsylvanian" Period.