Joseph R. Rohrer
Plants large to robust, forming loose wefts, growth monopodial. Stems procumbent to erect-ascending, irregularly and sometimes sparsely branched to regularly pinnate; paraphyllia absent; pseudoparaphyllia lanceolate to narrowly lanceolate; rhizoids sparse, at tips of branches and sometimes along stems. Leaves oblong-ovate, ovate-lanceolate, or lanceolate; margins narrowly revolute almost to apex, serrate to serrulate in distal 1/2, nearly entire proximally; apex acuminate; costa single, extending 1/3--2/3 leaf length, often ending in a spine; median cells prosenchymatous, narrowly elliptic to linear-flexuose; alar cells small, quadrate to short-rectangular. Sexual condition dioicous. Seta elongate, smooth. Capsule suberect to horizontal; peristome double, exostome cross-striolate on abaxial surface, papillose distally; endostome with high basal membrane, segments broad, perforated along the keel by narrow slits, cilia 1--3, often fused. Calyptra cucullate, smooth, naked. Spores 10--17 µm, very finely papillose.
Genus 1, species 1: widespread in temperate and boreal regions.
As originally described, the family had eight genera, but as recently circumscribed by W. R. Buck and D. H. Vitt (1986) and N. Nishimura et al. (1984), the family is monotypic. Rhytidium is an anomalous genus without clear affinities. Although traditionally placed near Hylocomium, Rhytidiopsis, and Rhytidiadelphus, a cladistic analysis by W. R. Buck and D. H. Vitt (1986) placed it as the sister group of a large clade that includes such families as Amblystegiaceae and Brachytheciaceae, but not Hypnaceae or Hylocomiaceae.
Buck, W. R. and D. H. Vitt. 1986. Suggestions for a new familial classification of pleurocarpous mosses. Taxon 35: 21--60.