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Abstract
Recent discoveries of true Gymnostomum calcareum Nees & Hornsch. and G. viridulum Brid. in California, U.S.A., prompted a new look at the genus in America. Perichaetia borne laterally on short branches, perichaetial leaves strongly differentiated from the cauline, and bulging exothecial cells are critical new traits introduced here to distinguish both species from G. aeruginosum. All three species may have distal laminal margins bistratose in patches.