Stereodon callichrous (Bridel) Braithwaite
Plants medium-sized, 2--6 cm, branches 0.2--1 cm, regularly to irregularly pinnate, pale green to yellow-green, shiny, procumbent to ascending, firmly or loosely attached to substratum. Stems yellowish to green, hyalodermous, with central strand, pseudoparaphyllia lanceolate. Leaves of stem falcate to circinate secund ovate to ovate-lanceolate gradually narrowing to a slender acumen, 1.5--2 × 0.75--0.8 mm, somewhat curved distal to alar region, margins plane, often sharply toothed in acumen that tapers to a 1-seriate apex, costa short and double or obscure, median cells linear, 50--60 × 8 µm, basal cells shorter, broader and porose, yellowish to unpigmented; alar cells forming a well defined region of thin-walled, often somewhat inflated cells 1--3 cells high and bounded by quadrate to triangular cells above. Sexual condition dioicous; inner perichaetial leaves erect, oblong-lanceolate and finely pointed, plicate, toothed near apex, costa obscure. Seta 2--3.5 cm, red-brown, capsules red-brown, inclined, somewhat curved, 1.5--2 mm excluding conic operculum, annulus 1--2 seriate.
Sporophytes produced summer, capsules maturing June--July. Epiphytic or more usually terrestrial, forest, open terrain; 0--1500 m; Greenland; Alta., B.C.; Alaska, Wash.; Europe; Asia.
In North America Hypnum callichroum is frequent only near the Pacific coast, where it is relatively easy to determine. The presence of a hyalodermis and the distinct alar region of thin walled cells that are strongly differentiated from the adjacent cells are reliable characters. Material without sporophytes can, however, be troublesome. Epiphytic specimens frequently have sporophytes, while the terrestrial generally lack them. Although H. Ando has identified Arctic specimens as this species, I remain unconvinced. This species is most likely to be confused with H. plicatulum and H. holmenii.