Trematodon Michx.

Plants mostly small, gregarious on soil. Stems unbranched or branching by innovation, beset below with smooth brown rhizoids, in cross-section with a central strand. Leaves erect-spreading, sheathing below, mostly abruptly shouldered and tapered from a ± oblong base to an elongate subula that is often equal or greater in length than the base. Laminal cells of leaf base ± oblong, thin-walled, non-porose, and smooth in N.Z. species; alar cells not differentiated. Costa often filling the subula, often poorly defined in leaf base, in cross-section (in lower subula) with a median band of guide cells.

Autoicous or dioicous. Perichaetia terminal. Perigonia with inner bracts reduced and pigmented, surrounding several antheridia and filiform paraphyses. Setae elongate; capsules erect or curved, with a ± cylindric or obovoid urn and an elongate and narrow neck, operculate (dehiscent), not sulcate, often strumose; exothecial cells firm- or thick-walled, not in distinct bands; stomata numerous and superficial, restricted to neck; annulus vesicular in N.Z. species; operculum long-rostrate from a conic base. Peristome variably developed, sometimes absent or nearly so, the teeth (if present) often split into unequal branches that are fused apically, and sometimes perforate. Calyptra smooth and inflated, cucullate, or rarely ± mitrate. Spores subreniform or round.

A moderately large genus predominantly distributed in the southern hemisphere. Magill (1981, p. 111) considered the genus to contain c. 82 species. The genus is in need of large-scale taxonomic revision, which would likely result in a substantial reduction in the number of recognised species. The species are defined primarily using sporophytic characters and are rarely collected when sterile. Rod Seppelt (pers. comm., 14 Oct. 2015) has suggested that costal anatomy may be of taxonomic value regionally. Three species are accepted in the N.Z. flora.

The genus was divided by Brotherus (1924, p. 174) into two subgenera based on the presence (subgen. Trematodon) or absence (or near absence) (subgen. Gymnotrematodon) of a peristome. Sainsbury (1955) emphasised calyptra form to define species regionally, but two of the three N.Z. species have calyptrae that vary in form.