STEMS: monomorphic, not readily fragmenting when dry, the branch tips appearing more or less curled when dry, prostrate, usually forming small loose low mats. RHIZOPHORES: produced throughout. LEAVES: relatively strongly dimorphic, the tip sharply pointed, with a stiff straight white glabrous seta 0.1–0.3 mm long (theis sometimes breaking off with age), the margins ciliate; those of the adaxial stem side 1.9–2.3 mm long, narrowly lanceolate, green to yellowish green when hydrated, the base abruptly adnate (differing in color from the stem); those of the abaxial stem surface 2.0–2.5 mm long, lanceolate to narrowly lanceolate, persistently gray to brown, the base decurrent (similar in color to the stem). STROBILI: 4–10 mm long, the sporophylls ovate-deltate, narrowed to an acute tip lacking a seta, the margins denticulate to short-ciliate. MEGASPORES: finely and often faintly rugose-reticulate, orange or yellow. NOTES: Rock ledges, cliffs, and open gravelly to rocky areas, usually on igneous or volcanic substrates or on sandstone: Gila, Graham, Maricopa, Pima, Pinal, Yavapai cos. (Fig. 1A); 600–1700 m (2000–5600 ft); AZ, TX; nw Mex. The taxonomy of S. arizonica is still not fully understood. In the desert mountains of western Pima County uncommon apparent hybrids occur between S. arizonica and the closely related S. eremophila. Some of these putative hybrids are sterile with abortive spores, but other plants intermediate for one or more morphological traits produce apparently well-formed spores that are slightly larger than those of either of the parents. These may represent an allopolyploid that should be recognized taxonomically. Where S. arizonica co-occurs with the morphologically distinctive S. rupincola, occasional putative hybrids also are encountered with intermediate morphology and abortive spores. REFERENCES: Yatskievych, G. and M. D. Windham. 2009. Vascular Plants of Arizona: Selaginellaceae. CANOTIA 5 (1): 39–48.