Acidodontium, with 11 species (Ochi 1992), is restricted to the neotropics. The gametophytes of Acidodontium are nearly inseparable from some large species of Bryum or Brachymenium. The genus is based on sporophytic features. Its capsules are generally erect and have long, slender, abruptly constricted necks, the operculum is hemispheric and only occasionally apiculate. Its exostome teeth are broad in the lower _ then abruptly constricted to slender filaments, and the dorsal (outer) trabeculae project laterally, i.e., in the direction of the adjacent teeth. The endostome in Acidodontium has a high basal membrane with broad segments that are completely split along the keel and laterally divergent in the direction of the adjacent segments. Although the above suite of sporophytic characters neatly circumscribes the genus, a related form of its odd endostome is found in Brachymenium columbicum where the segments are split along the keel and divergent, but in this case each ½ segment is fully united with the adjacent cilia and ½ segment. An endostome nearly identical to that of Acidodontium is also found in the Bartramiaceae (Bartramia, Breutelia, Philonotis, and Plagiopus).