Plants prostrate, annual, very small, without erect or ascending stems. Rhizophores present along basal branches. Stems widely branching with main segments 1-3 cm, 2-2.5(-3) mm wide across microphylls, decumbent or creeping. All leaves conspicuously dimorphic, arranged in 4 ranks (2 dorsal and 2 ventral), vein single, prominent on ventral side, usually not reaching apex, margin with 1 or 2 rows of specialized, elongated cells. Axillary leaves present at branch forks, inserted at ventral side of stem, broadly ovate to elliptic, sparsely serrulate along margin. Dorsal leaves ovate or broadly elliptic, 0.5-0.8 × 0.3-0.6 mm, base rounded or truncate, margin sparsely serrulate, apex acute to rounded. Ventral leaves broadly ovate or broadly oblong, rarely ovate, 1-1.3 × 0.7-0.9 mm, base rounded or somewhat cordate, margin usually sparsely serrulate but sometimes entire at basiscopic 2/3, apex rounded or broadly acute. Strobili terminal, complanate, 3-10 × 2.5-3 mm; sporangia on ventral side only, megasporangia at basal parts and microsporangia more apically; dorsal sporophylls significantly larger than ventral ones, widely spreading, broadly falcate, 1-1.3 × 0.5-0.7 mm, sparsely serrulate along margin, apex acute to rounded, single vein bearing a keel-like wing along its basal half on ventrally facing adaxial side, margin of wing toothed; ventral sporophylls pointing forward, shovel-like, ovate or broadly lanceolate, 0.7-0.9 × 0.4-0.5 mm, rounded at base, serrulate along margin apically but toothed nearer base, apex acuminate or acute, vein prominent on ventrally facing abaxial side.
Selaginella devolii most resembles young sporophytes of S. leptophylla. However, the latter has oblong ventral trophophylls and lanceolate dorsal sporophylls that easily distinguish it. Selaginella ciliaris, which usually grows together with this new species, is distinguished by its trophophylls and sporophylls with ciliate margins.
C and S Taiwan.
On moist rocky slopes with some shade; 600-1200 m.