The Pennsylvanian or Late Carboniferous extends from the end of the Mississippian, about 318 million years ago, to the start of the Permian 299 million years ago. This short length of time, only 19 million years, is one rationale for including the Pennsylvanian and Mississippian together as one period, the Carboniferous. But, there was a lot going on and the rock record is extensive, so we’ll let it go. In any case it’s the nomenclature that is used in the US.
Subdivisions, or ages, of the Pennsylvanian in the United States are named, from oldest to youngest, Morrowan, Atokan, Desmoinsean, Missourian, and Virgilian, each of which is represented by relatively distinct packages of rocks. Each age spanned about 6 to 8 million years. The international subdivisions of the Late Carboniferous are named for localities in Russia, where rocks of this time are well exposed. The ages are Bashkirian, Moscovian, Kasimovian, and Gzhelian, again from oldest to youngest. There isn’t a one-for-one correlation between the age names in the U.S. and in the Russian terminology, which obviously can confuse things sometimes.
—Richard I. Gibson
Time scale from Wikipedia