Tridontium Hook.f.

The following generic description and treatment of T. tasmanicum are based on drafts prepared by A.J. Fife.

Plants minute to robust, dark brown-green to yellow-green, or red-brown, gregarious or in turves on soil or rock. Stems simple or branched; central strand mostly absent in cross-section, but variably and often strongly developed in T. tasmanicum, sclerodermis present or absent. Leaves few to many, erect spreading or recurved when moist, variously altered when dry, ligulate to lingulate, broadly channelled with apex rounded, flat to cucullate; margins plane or recurved, unistratose, entire or crenulate by bulging cells with or without rounded papillae, unbordered or with a basal intramarginal border (in T. tasmanicum); upper laminal cells clear in outline, isodiametric with rounded lumina, firm- to thick-walled, smooth or with simple papillae; lower laminal cells rectangular. Costa concolorous, failing shortly before the leaf apex, protruding abaxially; adaxial superficial cells quadrate or elongate. Axillary hairs of 3–6 cells, with basal 1–2 cells brown. Brood bodies absent. Laminal KOH colour reaction yellow to yellow-orange.

Dioicous. Perichaetia terminal. Perigonia terminal or on very short branches. Setae elongate. Capsules erect, ellipsoid to cylindric, or turbinate (in T. tasmanicum). Operculum rostrate from a conic base, or mammillate (in T. novae-zelandiae). Peristome various (including lacking). Spores smooth.

Tridontium was long regarded as a monotypic genus, placed in subfamily Trichostomoideae of the Pottiaceae by Brotherus (1924). Zander (1993, p. 276), in his influential treatment of the family, suggested that the taxon might be a bridge between Pottiaceae and Grimmiaceae, sharing some features with the predominantly northern hemisphere grimmiaceous genus Scouleria. Buck & Goffinet (2000) subsequently transferred Tridontium tasmanicum to the Scouleriaceae. Later molecular studies found the species to be nested within the Pottiaceae in the general relationship of Didymodon and Leptodontium, based on rps4 gene sequences (Hedderson et al. 2004). Tridontium and Didymodon rigidulus (type species of Didymodon) were subsequently resolved as sister taxa (Cox et al. 2010), and a molecular analysis of 86 species of Didymodon and allied genera, together with morphological data, demonstrated a close relationship between these taxa and Tridontium tasmanicum (Jiménez et al. 2022). Two species endemic to N.Z. were included in the latter analysis, and were transferred to Tridontium as T. weymouthii and T. novae-zelandiae; these species placements are accepted here. A fourth species, also endemic to N.Z., has subsequently been described as Tridontium milleneri (Beever et al. 2023).

Perhaps not surprisingly, nomenclatural confusion has occurred between Weissia lancifolia var. weymouthii (R.Br.bis) Wijk. & Marg. (syn. Tridontium cockaynei​​​​​​​) and Weissia weymouthii var. lancifolia (R.Br.bis) Dixon. (syn. T. tasmanicum), a nom. illeg.

A unifying character of Tridontium is the nature of the leaf apex, rounded with a failing costa. The preference for wet habitats is also a feature of the genus. Typically stem cross-sections lack a central strand, but this is a variable feature in T. tasmanicum​​​​​​​.

The genus is unusual in Pottiaceae in that its members are hygrophilous, often being found on irrigated banks. Tridontium tasmanicum​​​​​​​ may, in addition, be truly aquatic.

Tridontium tasmanicum var. angustatum Dixon, a nom. nud., was placed in synonymy of Eucladium irroratum by Dixon (1923​​​​​​​, p. 122).