Authors: Sharon E. Bartholomew-Began

Plants 0.6 -- 2.4 cm, caespitose, unbranched or branched sparingly; shoot apex falciform. Axis slightly flattened on dorsal side, 0.1--0.3 mm wide, mostly thin and flaccid. Dorsal wing 2.0--3.0 mm wide, margin entire, lacerate or erose with age, with the wing cells near axis 86--113 ´ 22--50 µm, with the marginal cells 25--48 ´ 25--35 µm. Oil cells 20--24 ´ 20--25 µm; oil bodies 15.0--17.5 µm. Lateral leaf scales remote, oriented obliquely, 1-stratose, linear-lanceolate, 500--730 ´100--160 µm. Ventral leaf scales 1-stratose, linguiform, lanceolate, or linear, often with median constriction, 250--400 ´ 160--180 µm. Specialized asexual reproduction unknown. Sexual condition monoicous, protandrous. Antheridia 1--7(13) in a single series; antheridial body ca. 120--160 ´ 90--128 µm. Archegonial involucre ovoid, 1.4--2.0 ´ 1.0--1.5 mm, with the mouth contracted and often subacute, lamellae 8, longitudinal, 0.1--0.2 mm broad, with the margin undulate-sinuate or subentire. Sporophyte foot nearly spherical, ca. 160--180 µm; seta 100--115 µm; capsule ± globose to ovoid, 750--800 µm. Spores ovoid, 80--120 µm, with the distal face spinose, the spines 6--13 µm, the apices truncate, slightly dilated, occasionally emarginate-2-fid, rarely acute, the basal membranes forming a few imperfect areolae, with the proximal face spinose, the spines truncate or obtuse, or with tubercules, ca. 2--5 µm, basal membranes absent.

Disjunct and sporadic in arid and semiarid regions; low elevations; Calif.; Eurasia; Africa.

Riella affinis is known only from a single location in North America; Lake Lagunita, on the campus of Stanford University, Palo Alto, California. The unusual distribution of R. affinis suggests that it may not be native to North America, but instead introduced either inadvertently or with purpose to the Stanford University campus (perhaps by D. H. Campbell). In addition to differences in involucre morphologies, the absence of gemmae in R. affinis is often cited as a useful character to differentiate between R. affinis and R. americana.