Radula Dum., Comm. Bot., 112, 1822; Rec. d'Obs., 1835.

Martinellius S. F. Gray, Nat. Arr. Brit. Pl., 1, 690, 1821.

Stephanina O. Kuntz, Rev. Gen. Pl., 839, 1891. Schiffn. in Engl. & Prantl, Nat. Planfz., 1895.

Plants usually dioicous, may be autoicous or paroicous, medium, rarely small, in various shades of green, from yellowish to dark, often imparting a yellow-green stain to paper when wet, usually creeping and forming depressed mats on bark, earth, rock, mosses or filmy ferns. Stems usually pinnately to bi-pinnately branched, branches infrafoliar in origin. Leaves incubous, alternate, entire or toothed, complicate-bilobed; ventral lobe smaller, appressed to the underside of the dorsal lobe, or with the carinal portion inflated, rhizoids often arising from a mamilliform protuberance. Stipules everywhere absent. Involucral leaves usually larger than the foliage. Perianths usually acrogynous, frequently with 1–2 floral innovations, dorsiventrally compressed, plicate or smooth, oblong or with an elongated neck; lips truncate, crenate, undulate, or with a median cleft, margins sometimes winged. Capsule generally oval-cylindrical, valves with 2 layers of cells, dehiscing to the base. Androecia terminal or intercalary, bracts in pairs from 3 to about 30, antheridia usually single. Vegetative reproductive bodies present or absent.

A fairly large genus, with numerous tropical species.

Castle (1937) has divided the sub-genus Acroradula Spruce into 13 sections, using where fitting, the limitations and names of 4 of Stephani's 7 sub-genera, and regrouping and renaming more satisfactorily, the remaining 3.

The distinguishing features of the sub-genera and sections are as follows, the following species having been placed in their respective sections on the authority of the writer:—R. papulosa, sainsburiana, buccinifera, plicata, levieri, silvosa, allisonii.