Article: Silurian monograptids from Illinois
Six species of Monograptus are recognized from five localities in and adjacent to Illinois. The basinal lithologies of the Moccasin Springs Formation in southern Illinois and south-east Missouri contain M. roemeri, M. colonus, M. varians and M. varians var.pumilus?, M. dubius, and M. butovicensis, and the inter-reef lithologies in central and northern Illinois contain M. bohemicus, M. cf. M. dubius, and M. colonus. These species are indicative of the monograptid zones that characterize the Lower Ludlow Shale in Great Britain. On the basis of these assemblages, the upper part of the Niagaran Series in the central states region, the Racine Formation, the upper part of the Moccasin Springs Formation, the Mississinewa Shale of Indiana, and the Henryhouse Shale of Oklahoma are believed to be correlative to the Lower Ludlovian of Great Britain. The accepted correlation of the reef-bearing Racine Formation with the Lockport Group of New York State suggests that the Lockportian Stage of the Niagaran Series in its type region is largely, if not entirely, equivalent to the Lower Ludlovian of the type Silurian strata of Great Britain.