The Glen Dean Limestone, which lies at the top of the middle Chester (Homburg Group) in the Illinois basin, contains a crinoid fauna of 23 species. On the basis of general comparison with crinoids of British Carboniferous rocks and indirect correlation by way of Oklahoma and Arkansas stratigraphy, the Visean-Namurian boundary (P 2 -E 1 ) is placed in the middle Chester in the type area.

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