Smithsonian Misc. Collect. 72: 3–5. 1920.
Plants on rock or terrestrial, forming dense mats. Stems not readily fragmenting, prostrate, upperside and underside structurally different, irregularly forked; branches determinate, tips upturned. Rhizophores borne on upperside of stems, throughout stem length, 0.2 mm diam. Leaves conspicuously dimorphic, in 8 ranks, tightly appressed, ascending, green; abaxial ridges present; apex with deciduous, twisted, transparent bristle ± 0.3 mm, becoming acute to slightly mucronate in oldest branches. Underside leaves lanceolate to lanceolate-elliptic (on central ranks) or falcate (on marginal ranks), 2–2.7 × 0.5–0.7 mm; base decurrent, glabrous; margins ciliate, cilia transparent to opaque, spreading, 0.04–0.1 mm. Upperside leaves lanceolate, 1.3–1.4 × 0.3–0.4 mm; base abruptly adnate, pubescent, hairs often running along groove; margins ciliate, cilia transparent to opaque, spreading, ca. 0.1 mm. Strobili solitary, 3–8 mm; sporophylls ovate-deltate, abaxial ridges not prominent, base glabrous, margins ciliate, apex acute to mucronate.
Habitat: Rocky and sandy slopes, in open rock or crevices or in soil
Elevation: 130–1000 m
Ariz., Calif., Mexico in Baja California
Selaginella eremophila is most closely related to the Mexican S. parishii L. Underwood and S. landii Greenman & Pfeiffer. In S. eremophila and the following two species, S. arizonica and S. peruviana, the leaves are arranged in 8 conspicuous ranks: 3 underside (2 marginal, 1 central), 2 lateral, and 3 upperside (2 marginal, 1 central).
"thick" is not a number.