Plants: 1–4(–8) cm, light green or yellow-green. Leaves: oblong-ovate, concave, 1–2 × 3–5 mm; margins entire; apex abruptly subulate; costa vanishing in subula; distal laminal cells rectangular to oblong-hexagonal, 20 × 35 µm. Sexual: condition autoicous. Seta: stramineous, usually dark red with age, 1–5 cm. Capsule: not cleistocarpous, red, dark or black with age, long-ovate; hypophysis somewhat wider than urn distally; stomata over whole of hypophysis; operculum bluntly conic. Calyptra: conic-mitrate. Spores: 9–12 µm, smooth or slightly papillose. Phenology: Capsules mature summer.
Dung of carnivores, old bones, owl pellets, dry alpine, boreal, arctic habitats. low to high elevations. Greenland, Alta., B.C., Man., N.B., Nfld. and Labr., N.W.T., N.S., Nunavut, Ont., Que., Sask., Yukon, Alaska, Maine, Mich., Minn., N.H., N.J., N.Y., Oreg., Vt., Wash., W.Va., n Europe, Asia.
Tetraplodon mnioides is much more common than other species of the genus and is readily distinguished; the plants differ from those of T. angustatus by their larger tufts and longer sporophytes, which are dark red, becoming black with age. Sterile plants of T. mnioides differ from those of T. angustatus by their oblong-ovate, abruptly subulate leaves with entire margins, whereas those of T. angustatus are oblong-lanceolate, gradually subulate, and serrate. The distal laminal cell walls of T. mnioides are rather thin.
Plants: 1–4 cm, yellow-green distally. Leaves: ovate, very concave, 1.5 × 0.8 mm; margins entire; apex broadly acute; costa ending before subula; distal laminal cells rectangular, 25–30 × 15–18 µm. Sexual: condition autoicous or dioicous. Seta: stramineous, dark red with age, 0.3–0.7 cm. Capsule: not cleistocarpous, dark reddish, cylindric, somewhat oblong; hypophysis as wide as or wider than urn; stomata over whole of hypophysis; operculum hemispheric. Calyptra: cucullate. Spores: 10–11 µm, smooth or slightly papillose. Phenology: Capsules mature late summer.
Dung of carnivores, old bones, owl pellets in dry, very exposed places, open tundra, mountain summits. low to high elevations. Greenland, Alta., B.C., Man., N.W.T., Nunavut, Que., Yukon, Alaska, c Europe, e, s Asia.
Tetraplodon urceolatus is an arctic or alpine moss very similar to reduced northern forms of T. mnioides; however, T. urceolatus has short yellowish setae, small strongly concave appressed leaves, and a short hypophysis about as long as the urn, sometimes lighter, and the same color or darker than the urn with age, and with relatively fewer stomata generally confined to the distal portion of the hypophysis. In general, T. urceolatus has firm leaves with stiff, rounded apices and thick distal cell walls. The exostome teeth of T. urceolatus are reflexed, orange-red at the apices, and inserted below the capsule mouth.