Plants terrestrial or on rock, forming clumps. Stems radially symmetric, underground (rhizomatous) and aerial, not readily fragmenting, irregularly forked; rhizomatous and aerial stems often with 1 branch arrested, budlike, tips straight; rhizomatous stems mostly ascending; aerial stems erect or ascending. Rhizophores borne on upperside of stems, restricted to rhizomatous stems, 0.2--0.33 mm diam. Leaves dimorphic, in pseudowhorls of 4. Rhizomatous stem leaves persistent, appressed, scalelike. Aerial stem leaves tightly or somewhat loosely appressed, ascending, green, narrowly triangular-lanceolate or narrowly lanceolate, 2--3 X 0.4--0.5 mm; abaxial ridges present; base cuneate, strongly decurrent, pubescent or glabrescent; hairs restricted to base; margins short-ciliate, cilia transparent, scattered, spreading at base, dentiform and ascending toward apex, 0.02--0.07 mm; apex plane, attenuate; bristle white to whitish, straight, coarsely puberulent, 0.25--0.85(--0.9) mm. Strobili solitary, (0.5--)1--3(--3.5) cm; sporophylls ovate-lanceolate to lanceolate, often abruptly tapering toward apex, abaxial ridges not prominent, base glabrous, rarely with few hairs, margins ciliate, apex often recurved, bristled.

Selaginella arenicola and related species have been considered as forming a species complex. This interpretation has been the center of much taxonomic controversy (R. M. Tryon 1955; G. P. Van Eseltine 1918). Tryon recognized one species in the complex, S . arenicola , with three subspecies: subsp. arenicola , subsp. riddellii , and subsp. acanthonata . Other authors (e.g., R. T. Clausen 1946) treated the subspecies as species. I recognize two well-defined species within this complex, S . arenicola and S . acanthonota , which are readily distinguishable by the characteristics given in the key. Some specimens reported by R. M. Tryon (1955) as intermediate between S . arenicola and S . acanthonota appear to be hybrids between S . acanthonota and S . rupestris . In particular, more detailed studies are needed to assess whether populations from Georgia are hybrids or variants of S . acanthonota or of S . rupestris . Future studies are also needed to determine relationships and proper taxonomic rank of Selaginella arenicola subsp. arenicola and subsp. riddellii , which are provisionally recognized here.