Authors: Sharon E. Bartholomew-Began

Plants 1.0--3 cm, caespitose, unbranched or dichotomously branched 1 to 4 times; shoot apex circinate. Axis elliptic, 0.2--0.8 mm diam. Dorsal wing 1.5--5 mm wide, margin entire, lacerate or erose with age, with wing cells near the axis 70--100 ´ 30--50 µm, with the marginal cells 30--50 ´ 22--40 µm. Oil cells 21--23 ´ 17--25 µm; oil bodies 15--17 µm diam. Lateral leaf scales remote, oriented obliquely, 1-stratose with a multistratose base, ovate to oblong (obovate to lingulate), 355--800 ´ 125--400 µm. Ventral leaf scales 1-stratose, irregular, often panduriform with a broad isthmus and equal or unequal lobes or oblong (obovate-ovate), 165--540 ´ 100--350 µm. Specialized asexual reproduction by gemmae, ventral from the youngest axis portions, intermixed apically with the scales, panduriform with two rounded lobes, 730--1050 ´ 450--775 µm, multicellular. Sexual condition dioicous. Antheridia up to 75 in a single series; antheridial body ca. 210--235 ´ 160--185 µm. Archegonial involucre subglobose to ovoid, 1.4--1.8 ´ 0.8--1.2 mm, with the mouth truncate or slightly pointed, subpapillose; lamellae absent. Sporophyte foot broadly ovate, ca. 130-- 250 ´150--200 µm; seta 100--200 µm; capsule globose, 0.5--1.3 mm. Spores spherical, 100--130 µm, with the distal face spinose, the spines 10--24 µm, occasionally curved, the apices somewhat truncate, dilated, acute, the basal membranes irregularly radiating, with the proximal face spinose, the spines conical, 3--6 µm, basal membranes rare or absent.

Submerged aquatic in temporary pools and streams in arid and semiarid areas; moderate elevations; Calif., N.Mex., S.Dak., Tex.; Mexico; South America.

Riella americana is known from Texas, New Mexico (San Miguel County and Luna County), California (Modoc County), and one location in South Dakota (near Brookings).