Paul L. Redfearn, Jr.
Plants very small to robust, in loose to dense, soft, shiny, green, yellowish, golden, or reddish tufts. Stems erect, ± straight to distinctly secund, not to sparsely branched, pseudoparaphyllia lacking, epidermal cells in 3--4 rows of small, very thick-walled cells surrounding a zone of larger, thinner-walled cells, and a central strand composed of very thin-walled cells; branches radiculose at base. Stem and branch leaves similar, erect to erect-spreading, lanceolate to ovate, acuminate, ± concave, plane to strongly plicate; margins incurved from base to middle, plane or recurved above, acumen entire or minutely serrulate; costa lacking or short and double to reaching the middle of the leaf, one fork usually longer than the other; costa cells similar to leaf cells but thick-walled and porose; distal leaf cells linear, ± flexuose, smooth, usually somewhat incrassate and porose throughout, but conspicuously so in proximal portion of leaf; alar cells not differentiated, cells across insertion often darker, relatively short and thick-walled. Specialized asexual reproduction very rare, by axillary propagula. Sexual condition dioicous. Perigonia and perichaetia near base of stem, perichaetial leaves lanceolate, ovate-lanceolate to ovate-triangular, acuminate, margins plane, serrulate to sharply serrate, distally, ecostate, cells linear, thick-walled, porose. Seta elongate, golden brown, purple or reddish, twisted. Capsule erect and symmetric to slightly asymmetric, cylindric to subglobose, commonly flaring at mouth; operculum conic; exothecial cells ± isodiametric to rounded; stomata few at extreme base of urn; annulus composed of 2--3 rows of ± thick-walled cells; peristome double, attached at the mouth, exostome teeth broadly lanceolate, bordered, pale yellow, papillose distally, cross-striolate proximally, segments keeled and perforate, front surface with a zig-zag center line; basal membrane of endostome low to moderately high, basal segments broad to linear, usually papillose, keeled and perforate, as long as exostome to somewhat longer, cilia present or lacking. Calyptra cucullate, smooth and naked. Spores 10--16 µm.
Species 10 (5 in the flora); terrestrial, calcicolous habitats from near sea level to high elevations in temperate to boreal and Arctic regions; North America, Europe, Asia, Africa.
Many authors (V. F. Brotherus 1907; E. Lawton 1971; A. Noguchi 1990; E. Nyholm 1965) have placed Orthothecium in the Entodontaceae. However, W. R. Buck (1980) concluded that Orthothecium, based on leaves with a short, double costa, undifferentiated alar cells and a hypnoid peristome, should be placed in the Hypnoideae of the Hypnaceae
Brotherus, V. F. 1907. Entodontaceae. In: A. Engler & K. Prantl, eds., Die naturlichen Pflanzenfamlien, 1(3): 870--899. Leipzig. Buck, W. R. 1980. A generic revision of Entodontaceae. Jour. Hattori Bot. Lab. 48: 147--150. Lawton, E. 1971. Moss Flora of the Pacific Northwest. Miyazaki, Japan. Noguchi, A. 1994. Illustrated Moss Flora of Japan. Part 5 Miyazaki, Japan. Nyholm, E. 1965. Illustrated Moss Flora of Fennoscandia II. Fasc. 5, 565--570.
Ireland, R. R. 2003. Isopterygiopsis. Bryophyte Flora of North America, Provisional Publication, Version 2. BFNA Web Site, Missouri Botanical Garden.