Three cores from the Deadwood Formation in the Williston basin of Montana have furnished trilobites and brachiopods referable to basal Ordovician Zones A, B, and possibly C of R. J. Ross, Jr. (1951) and L. F. Hintze (1952). The fossils occur in dark green micaceous, pyritic shales interbedded with a few glauconitic limestone pebble conglomerates. Zone A faunules are characterized by long-ranging graptolites and brachiopods with trilobites of Symphysurina species and Euloma cordilleri n. sp. Zone B faunules, in over 100 ft of section, contain many of the trilobites Clelandia, Bellefontia, and Xenostegium, and of Lingullela; a new problematica, Huishea flexa, is rare. A few specimens of Apheoorthis and Symphosurina appear 25 ft higher, tentatively Zone C.

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