Dioicous, autoicous or synoicous. Plants robust, perennial, with stoloniferous stems matted with rhizoids and erect simple or forked reproductive shoots. Rhizoids brown, papillose. Leaves on stolons small, crowded. Leaves on reproductive shoots small and remote at the base, larger and forming a comal tuft or rosette at the apex, elliptic to suborbicular; apex rounded to mucronate; costa single, strong, ending in the apex or just below; leaf bordered by linear cells occasionally with weak rounded teeth. Laminal cells rounded-quadrate to hexagonal, mostly thick-walled, non-porose or porose, becoming larger, longer and more rectangular towards the costa and insertion.

Perigonia and perichaetia terminal. Perigonia usually discoid, with large perigonial leaves. Perichaetial leaves forming a comal tuft of narrow and erect leaves; inner ones small and narrow. Setae solitary or polysetous 1–4, clustered, short or long. Capsules exserted, inclined or pendant, rarely erect, oblong-ovoid to cylindrical; neck short; annulus present; operculum convex to obliquely rostrate. Peristome usually well developed; exostome teeth 16, lanceolate-acuminate, trabeculate or not; endostome segments alternating with and similar in length to teeth, rarely reduced; basal membrane high, rarely reduced and low; cilia usually well developed, nodose. Spores papillose, comparatively large.

There have been extensive cytological studies on the Mniaceae summarised by Fritsch (1991) and Newton (1986) under individual genera. For Plagiomnium the recorded chromosome numbers are n = 6, 7, 12, 14 and 21 with karyotype data for many, while for Orthomnion n = 14 has been reported by Ono for a single species in Japan (Fritsch, 1991).

Native.

Mniaceae Schwägr., Sp. Musc. Frond. 25 (1830). Type: Mnium Hedw., nom. cons.

Koponen (1988) divided the Mniaceae sens. lat. into three families, viz. Mniaceae sens. str., Plagiomniaceae and Cinclidiaceae. In the classification scheme of Buck & Goffinet (2008) several genera (e.g. Pohlia, Mielichhoferia and Schizymenium), traditionally included in the Bryaceae, but phylogenetically closer to the Mniaceae (Cox & Hedderson, 2003), were transferred into the Mniaceae. However, more recent molecular evidence from nuclear, chloroplast and mitochondrial genomes suggests that a broad Mniaceae clade may have separated early into two diverging lineages, the families Mielichhoferiaceae and Mniaceae. Thus, Pohlia, Mielichhoferia, Schizymenium and other genera have now been recognised as constituting the Mielichhoferiaceae (Shaw, 2009; Goffinet et al., 2012), while 12 other genera, including Mnium, Plagiomnium and Orthomnion, are retained in the Mniaceae. We accept the more recent interpretation for Australian mosses, recognising the two families with Mielichhoferia and Pohlia in the Mielichhoferiaceae and Orthomnion and Plagiomnium in the Mniaceae.

In Australia sporophytes are rare in Plagiomnium and have not yet been found in Orthomnion. Vegetative plants of Orthomnion can be distinguished from Plagiomnium by the more densely tomentose stolons and bases of erect stems, an entire leaf margin, leaves that are more closely spaced in two rows, a weaker costa, the lack of stereid bands, and the thin-walled and porose laminal cells. The presence of porose cell walls is not usually emphasised in descriptions of Orthomnion but is shown clearly in published illustrations [e.g. Koponen (1980a: 43, figs 16–20, 22–24); Koponen (1982a: 79, fig. 3); Eddy (1996: 197, fig. 465)]. Subsection Orthomniopsis (Broth.) T.J.Kop., to which the Australian species O. elimbatum belongs, is characterised by short setae (less than 10 mm long) and trabeculate peristomes in contrast to the other subsection Oligodon T.J.Kop., not present in Australia, which has longer setae and reduced peristomes.

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