A. oerstedianum (C. Müll.) Mitt., J. Linn. Soc, Bot. 12: 605. 1869.
Catharinea oerstediana C. Müll., Syn. Muse. Frond. 2: 558. 1851.
Atrichum undulatum (Hedw.) P.-Beauv. var. oerstedianum (C. Müll.) Crum, Bryologist 74: 166. 1971.
Evidently uncommon in Mexico, judging by the fact that no specimens of A. oerstedianum were found among more than SO Atrichum collections from 10 states throughout Mexico. This species evidently becomes more common southward in Guatemala and Costa Rica, the type locality. According to Ireland (1969), the robust, dioicous A trichum of eastern North America is this species.
Atrichum selwynii Aust. sensu Ireland, 1969, of western North America, has been collected in Guatemala (Dept. Huehuetenango, F. J. Hermann 26321a, MICH) and should be expected in Mexico. The leaves of this species are lingulate, acute to obtuse, with margins coarsely and often irregularly toothed, the teeth crowded near the leaf apex, and the cells of the lamina are relatively large (25-40 µm).