Plants with somewhat crenulate, undulate margins. Thalli 2--4 × 0.5--0.9 cm, elongate, slightly widened distally; epidermis persistent, smooth; air pores and areolation indistinct; epidermal cells 45--50 × 30 35 µm with small bulging trigones; air pores surrounded by 3 5 concentric rings of 6 8 cells each; ventral scales with decolorate margins and scattered oil cells; appendages 2 3 per scale, linear and hair like, 2 3 cells wide, ending in 1 3 single cells. Sexual condition paroicous (rarely polyoicous, but usually with some paroicous thalli in the mat); androecia reniform to lunate or reduced and chevron-like, positioned 1 10 mm posterior to the base of the gynoecial stalk; gynoecium in apical notch, stalk 1.5 3 cm, purplish; carpocephalum green, with air pores in the upper portion, 4 5 lobed below, each lobe directed downward, the slit widening to form a circular opening as the sporophyte expands. Sporophyte capsule pale greenish; Spores yellow, 65 80 µm; elaters 250 350 × 10 12 µm, 2 3 spiraled.

Capsules mature March--August. Soil over rock that is commonly but not exclusively calcareous, habitats that are at least periodically moist, temperate areas; subcosmopolitan: Alta, BC, Man., Ont, Que.; Ala., Ariz., Ark., Calif., Colo., Conn., D.C., Fla., Ga., Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Mont., Nebr., N.H., N.J., N.Mex., N.Y., N.C., Ohio, Okla., Oreg., Pa., R.I., S.C., Tenn., Tex., Utah, Va., Vt., W.Va., Wis.; South America; Europe; Asia; Africa; Australia; Pacific Islands.

Reboulia hemisphaerica, with its wide distributional range, exhibits considerable variation. From time to time these plants have been variously described as autoicous or dioicous as well as paroicous. The North American populations appear to be rather consistently paroicous, rarely failing to produce either androecia or gynoecia. When this failure occurs, some populations may appear to be autoicous or dioicous but paroicous individuals can usually be found within the mat of plants. Also, in Reboulia collections, the size of androecia decreases with age and, before elongation of the gynoecial stalk, androecia are quite turgid and bear marginal scales. As the gynoecial stalk elongates and sporophytes mature, the androecia shrink and marginal scales often fall away. Additional synonyms for the species are given by T. C. Frye and L. Clark (1937).

DOUBTFUL TAXA Reboulia hemisphaerica subsp. australis R. M. Schuster has been reported from Quebec, Massachusetts and Virginia. This name, based on New Zealand plants, is a synonym of R. queenslandica (Stephani) M. L. Hicks. The Australia and New Zealand populations are rather consistently autoicous, and no undoubted North American material has been located. For following two taxa, Reboulia hemisphaerica subsp. acrogyna (R. M. Schuster) R. M. Schuster (reported from Texas), R. hemisphaerica subsp. dioica R. M. Schuster (reported from North Carolina), material is not available, and the assigned characters do not justify taxonomic recognition.