Scopelophila cataractae (Mitt.) Broth., Nat. Pflanzenfam. 1(3): 436. 1902.
Weissia cataractae Mitt., J. Linn. Soc., Bot. 12: 135. 1869. Hyophila cataractae (Mitt.) Jaeg., Ber. Thätigk. St. Gallischen Naturwiss. Ges. 1871–72: 358. 1873. Merceya cataractae (Mitt.) C. Müll., Gen. Musc. Frond. 384. 1901 [1900]. Protologue: Ecuador. Andes Quitenses, ad rupes humidas secus cataractam Agoyan fluvii Pastasae (5000 ped.), Spruce, n. 45c.
Tortula williamsii Bartr., Bryologist 27: 70. 1924. Protologue: U.S.A. On ledges, White House Canyon, Santa Rita Mts., Arizona. E. B. Bartram, No. 907, February 7 1924, Altitude, 6500 ft.
Merceyopsis mexicana Bartr., J. Wash. Acad. Sci. 18: 577. 1928. Protologue: Mexico. On steep rock, San Sebastian, East of Segundo Arroyo Jalisco, Mexico, 1,500 meters, January 25, 1927, Mrs. Ynes Mexía, no 1568a.
Plants small, caespitose, green to yellowish green or brown with age, saxicolous or terricolous. Stems 5–20 mm high, irregularly branched, central strand absent, cortical cells thin-walled, hyaline, uniform throughout; rhizoids scattered on stem, smooth or verrucose. Axillary hairs 5–6 cells long, lower 1–2 cells brown. Leaves 1.5–2.5 mm long, ligulate to lingulate, erect to spreading at base, weakly keeled, flexuose when dry, erect when wet; apices acute to short-acuminate or rounded-obtuse; margins entire, plane above, recurved at base; costa narrow, subpercurrent to percurrent, surface cells rectangular, smooth on both surfaces, median guide cells and single (dorsal) stereid band well-developed, ventral surface layer of enlarged cells present; upper cells rounded-quadrate or oblate, 4–8 x 4–10 μm, firm-walled, smooth to somewhat bulging on both sides, basal cells rectangular, thin-walled, smooth, 14–30 x 12–16 μm, alar cells not differentiated. Dioicous. Perichaetia and perigonia terminal. Setae smooth, 2–5 mm long, yellowish brown. Capsules short-cylindrical, erect, 1.0–1.2 mm long, smooth, yellowish brown; exothecial cells quadrate to rectangular, thin-walled; stomata in neck; opercula rostrate, beak erect, 1 mm long; annuli persistent, clinging to the capsule mouth after dehiscence; peristome absent. Spores 12–14 μm, granulate. Calyptra not seen.