Mnium caespiticium var. warneum Röhling, Deutschl. Fl. ed. 2, 3: 94. 1813; Bryum warneum (Röhling) Bridel

Plants in dense or open turfs, red, green, or yellow-green. Stems 0.5-2(-3) cm, fertile stems comose, innovations evenly foliate. Leaves yellow-green to red, crowded, strongly contorted to shrunken when dry, ovate-lanceolate, flat, 0.5-2 mm, often gradually enlarged toward stem apex; base usually green, not decurrent; margins revolute proximally, limbidium strong, in 2 or 3 rows; apex acuminate; costa short-excurrent, awn slender; proximal laminal cells rectangular, 3-4:1; medial and distal cells 18-22 µm wide, 2-3:1, walls thin. Specialized asexual reproduction absent. Sexual condition usually polyoicous, sometimes autoicous. Seta purple or red, 2-4(-5) cm, stout, straight to somewhat flexuose. Capsule yellow-brown or brown, pyriform, symmetric, 2-4 mm, mouth yellow; ; operculum long-conic; peristome reduced; exostome teeth yellow or brown basally, pale yellow to hyaline distally, lamellae straight, pores absent near base along mid line; endostome weakly adherent to exostome, basal membrane 1/2 exostome height, segments narrowly perforate, cilia absent or rudimentary. Spores with size variable in same capsule, (32-)36-48(-50) µm, pale green or yellow.

Capsules mature Jul-Aug. Damp soil; low elevations (0-50 m); Greenland; Que.; n Eurasia (Himalayan Nepal).

Ptychostomum warneum is an arctic-boreal species that is distinguished from the related P. calophyllum by its pyriform brown capsule with a slender neck, generally larger spores, and acuminate leaves. The stems are sparsely radiculose; the costal awns are smooth. The species is known from only one location in Quebec, in the Mingan Islands. Capsules are needed for identification.