The Trenton (Middle Ordovician) is divided into three Prasopora zones: the lowermost has abundant P. simulatrix Ulrich; the middle has P. shawi n. sp., P. sardesoni n. sp. and P. sanfordi n. sp.; the uppermost has P. selwyni (Nicholson), P. occulata Foord, and P. affinis Foord. The zones are correlated with the Trenton of Minnesota. Monticules were absent from core specimens, so may be an artifact of weathering of surface specimens.