Among our collections were many specimens of Raddiella, a tropical American genus of monoecious olyroid grasses. These small grasses prefer sunny habitats of continuous moisture and are especially fond of wet cliff faces and rocks near waterfalls where they are exposed to a fine mist. The specimens which we collected on moist cliff faces matched others previously collected on Kaieteur Plateau and tentatively referred to R. malmeana, a species described from the type collected much farther south in Mato Grosso, Brazil. The material from Kaieteur Plateau, however, differs in several respects from the Brazilian specimens and is described below. Specimens growing near the splash-basin of Kaieteur Fall are unlike either of those mentioned above and represent a further undescribed species. A study of aU of the Raddiella specimens at the U. S. National Herbarium has turned up still another undescribed species, collected at Maipuri Falls in British Guiana.
Until now, the genus Raddiella comprised three species, two of which, R. nana (Doell) Swallen and R. malmeana (Ekman) Swallen, were transferred, respectively, from Raddia and Olyra by Swallen when he established the genus. The third, R. truncata, was described by him from specimens collected near Waratuk Falls in the Potaro River Gorge, British Guiana. Raddia hoehnei Pilger, based on Brazilian material, does not seem to differ significantly enough from extreme forms of Raddiella nana to warrant its recognition as a species. With the three new species described in this paper, the genus Raddiella is composed of six known species. Four of these are known only from habitats near waterfalls. It is plausible that these species are endemics and that additional species of the genus may be anticipated from the floras surrounding other waterfalls.