The Arrow Canyon section of the Bird Spring Group has been proposed as a stratotype section for the base of the Pennsylvanian and of the Permian because of its apparently uninterrupted deposition across those boundaries, extensive invertebrate fauna, nearly total exposure, and readiness of access. Both fusulinid zonation and detailed petrographic studies have already been

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completed. The Missourian and Virgilian sequence includes about 132 m of carbonate rocks divided into 46 units for collection and description. Eighteen of these units yielded a silicified brachiopod assemblage totaling 21 strophomenids and 17 taxa of other orders. Fossil abundance is greatest in the lower part of the Missourian and from the topmost Missourian through the middle Virgilian. Correlation with the middle Virgilian of North Texas and, perhaps, the lower Missourian of the southwest is especially strong. Notable range extensions include: (1) Paeckelmanella? n. sp., an otherwise Arctic Permian taxon, in the Virgilian; (2) Mesolobus euampygus in the Early Missourian, an extension of generic range from the Desmoinesian; (3) Paucispinifer n. sp., a Permian genus occurring in the issourian; and (4) Neospirifer latus lateralis, a Missourian species occurring in the Virgilian. In addition, Calliprotonia n. sp. is reported the second occurrence of this genus in the North American Virgilian.

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