Type: ARGENTINA. Buenos Aires: Isla Recreo, Parana River, Mar 1880, Schnyder s.n. (lectotype H).
The only material present of O. truncato-dentatum is a single stem apex with one not very well preserved capsule. The leaves are lanceolate to ovate-lanceolate with the margins broadly recurved almost the entire length and with apiculate apices. The seta is short. The capsule is almost smooth and with superficial stomata. The spores are fairly small, 15-22 µm in diameter.
It is not possible from this single stem apex to settle the taxonomic status of O. truncato-dentatum. The only species known from the area is O. araucarieti which, however, has acuminate leaf apices and capsules with diflferentiated exothecial cells. Orthotrichum paraguense, known to the north, is even more diflferent. Orthotrichum truncato-dentatum could thus be a distinct taxon, but its position can not be settled until more recent collections have been made. Unfortunately, Isla Recreo is today (C. Matteri, pers. comm.) a suburban area with numerous country clubs, in which no Orthotrichum would be expected.