diagnoSiS: Pleurothallis petroana is similar to P. sca-brilinguis, but the new species differs by an oblong-lanceolate dorsal sepal (vs. oblong-ovate), an ovate synsepal (vs. broadly ovate), linear, acuminate petals (vs. narrowly linear-triangular, acute) and a lip ovate-lanceolate, verrucose-papillose, slightly pilose (vs. ob-long-triangular, subacute or rounded, papillose), with a spathulate glenion (vs. obovate, slightly bilobed)
Plant 15 to 26 cm tall, epiphytic, occasionally ter-restrial, caespitose, erect to suberect. Roots slender, 1.3 mm in diameter. Ramicauls yellow-green, 12–24 cm long, with a sheath located at the first basal third and two other sheaths at the base, tubular, broad, papy-raceous, brown. Leaf light green, bright, coriaceous, minutely papillate, ovate-lanceolate, acuminate, 7–10 × 4.0–5.5 cm, the base sessile, cordate. Inflorescencea fascicle of successive single-flowers, rarely with two simultaneous flowers, enclosed at the base by a spatha-ceous bract ca. 12 mm long; peduncle terete, green, 9–11 mm long; floral bract tubular, papyraceous, acute, 9–11 mm long. Pedicel terete, up to 13 mm long. Ovary light green, papillate, pedicellate, cylin-drical, longitudinal sulcate, covered with black dots, 6 mm long. Sepals membranaceous, glandular-papillate, margins irregular. Dorsal sepal dark red, margins pale yellow, oblong-lanceolate, acute, 14–16 × 3.0–4.2 mm, 3-veined. Lateral sepals, connate into an ovate, acute synsepal, dark red with a central yellow stripe and pale yellow margins, concave at the base, 13.5–15.0 × 8.5–9.2 mm, 4-veined. Petals dark red, margins pale yellows, fleshy, linear, acuminate, microscopically verrucose-papillose on the dorsal surface, margins ir-regular, minutely denticulate, 7.8–9.0 × 1.2–1.5 mm, 1-veined. Lip yellow-red, fleshy, oblong-lanceolate, acuminate, verrucose-papillose, slightly pilose abaxi-ally, margins irregular, denticulate-ciliate, 7.5–8.6 ×3.4–4.2 mm, 3-veined, the disc with a small, spathu-late glenion near the base, the base truncate forming two pronounced lobes, subacute. Column light red, short, broad, semiterete, margins irregular, 1.5 × 1.2 mm, with a short, thick; stigma apical, bilobed. Anther cap light red, apical, obovate, minutely papillose, 0.6 ×0.5 mm Pollinia 2, yellow, obovoid. Capsule not seen.