Hypnum sect. Ptilium Sullivant [illegitimate name]
Plants medium-sized, forming loose mats of suberect to ascending shiny yellowish green to golden regularly pinnate shoots. Stems 3--10 cm, with branches 0.3--1.5 cm arising at right angles; main stem tips usually hooked at apex, branches hooked at tip or attenuate. Leaves of main stem squarrose, 2--3 mm, strongly plicate, broadly ovate, tapering to tip; branch leaves 1--2 mm, oblong-lanceolate, tapering to tip, usually circinate-secund, and pointing toward the base (not underside) of the main shoot, plicate, costa short and double, margins plane, toothed distally; cells smooth, long-linear and flexuose in lamina. Pseudoparaphyllia foliose. Specialized asexual reproduction lacking. Sexual condition dioicous. Seta smooth, red-brown, 2--3 cm. Capsule red-brown, arcuate, horizontal 1.5--2 mm, contracted below mouth, operculum long-conic, smooth, annulus narrow; peristome double, exostome red-brown, lanceolate, striolate basally, papillose distally; endostome pale yellow from a broad basal membrane; cilia 2 or 3, nearly equaling segments. Calyptra cucullate, smooth. Spores spherical, smooth to finely papillose, 12--16 µm.
Species 1 (1 in flora): North America, Europe, Asia. Terrestrial habitats in temperate, boreal and subarctic regions.
Ptilium is a remarkably elegant genus that is usually easily distinguished from Hypnum in the extremely plicate circinate leaves that curve toward the base of the main stem, not to the underside of the stem as in Hypnum. Plumose individuals of Sanionia are similar but this feature is also reliable, as is the double costa in Ptilium, which is single in Sanionia.
