Riccia frostii Austin

Riccia frostii Aust. Bull. Torrey Club 6: 17. 1875
Riccia Watsoni Aust. Bull. Torrey Club 6: 17. 1875. Riccia Beckeriana Stephani, Bull. Herb. Boiss. 6: 374. 1898.
Thalli small to medium-sized, 2-6 times dichotomous, forming compact rosettes 4-15 mm. in diameter, or at length irregularly gregarious, dark-green, grayish-green, or red-purple above, concolorous below, the dorsal surface obscurely reticulate or substriate in younger parts, soon becoming spongiose or lacunose, especially on drying; root-hairs (always?) smoothwalled; main segments obovate to linear, 0.5-2 mm. wide; terminal segments obovate, oblong, or linear, obtuse, obscurely emarginate; margins rounded-obtuse to subacute, naked! often red-purple, commonly plane; median sulcus usually well defined only at apex; scales, wanting or very rudimentary ; transverse sections oblong or subelliptic, mostly 1.5-3 times as broad as high, 14-20 cells thick in median parts, the ventral outline lightly convex; dorsal epidermis rather indeterminate, the cells of the primary stratum subconic, obtuse, or fiattopped, 40-70 /i broad, mostly broader than high, subpersistent or collapsing irregularly; air-chambers sometimes narrow and columnar, with occasionally transverse or oblique par titions, but, for the most part, elongate-polyhedral, separated by unistratose lamellae, and showing 2-4 vertical series in a transverse section, but only 1 or 2 series in longitudinal section. Dioicous; antheridial plants small, often red-purple; antheridial ostioles elevated 75130 ju; capsules usually numerous, their covering soon ruptured, exposing the spores; spores brown, 40-65 /* in maximum diameter, angular, with granulate-crenulate margin 2-3 /x wide,, the outer face often coUabent, marked with numerous short delicate wavy rarely anastomosing ridges, commonly showing in profile minute crowded obtuse or truncate papillae about 1 fj, high, the areolae when formed only 1.5-3 n wide, the inner faces similarly but a little less strongly marked.
Type locality: Nevada.
Distribution: Vermont to Montana, Washington, and California; Lower California; reported also from Europe and Asia.