Middle Silurian trilobites from Arkansas and Oklahoma, USA

Representatives of the Illaenidae, Bumastinae and Scutelluidae in the St. Clair Limestone of northern Arkansas and the Fitzhugh Member of the Clarita Formation of south-central Oklahoma, both units of Wenlock age, include 21 species belonging to Illaenoides Stenopareia Bumastus Cybantyx Liolalax Ciliscutellum?, Kosovopeltis Octobronteus? and Planiscutellum. At least two unnamed scutelluid genera are represented by indeterminate pygidia. New species are Octobronteus? incertus O.? ormistoni O.? petasus and Planiscutellum apollo, the last from the middle Llandovery (Aeronian) of Anticosti Island, Canada. Bumastus ioxus (Hall, 1867), a species widely recorded from the Silurian of North America, is regarded as a junior synonym of B. barriensis Murchison, 1839, type species of the genus. The trilobites support a Sheinwoodian age for the St. Clair Limestone