Orthodontium Schwägr.

Plants forming turves or cushions. Stems short or elongate, simple or branching by innovation, beset below with smooth, brown rhizoids. Leaves flexuose, linear-lanceolate, slenderly acute, unbordered, concave, entire. Mid laminal cells linear, smooth, firm-walled. Costa narrow, ending below the leaf apex or percurrent.

Variably monoicous. Setae elongate, slender, and ± flexuose; capsules erect or inclined, obovoid-cylindric, with a short or long neck, sulcate or smooth when dry; stomata superficial; annulus absent; operculum obliquely rostrate from a conic base. Peristome double, fragile; exostome teeth linear-lanceolate, papillose or smooth, with a nearly straight divisural line; endostome segments generally longer than teeth, linear, basal membrane and cilia absent. Spores spherical, medium-sized.

The erect or suberect capsules and the near or total absence of an endostomal basal membrane lend coherence to the genus. Orthodontium was placed in a small subfamily within the Bryaceae by Brotherus (1924), while Goffinet et al. (2009) place it in the Rhizogoniales in the general relationship of the Rhizogoniaceae and Aulacomniaceae.

Orthodontium is a genus of c. 12–14 species distributed on all continents except Antarctica. The genus was revised by Meijer (1952), whose taxonomic concepts were narrow. The Australasian taxa he recognised are differentiated by a suite of overlapping quantitative characters. Sainsbury (1955a, 1955b) applied these concepts to N.Z. material and concluded that two species occurred here: the widespread O. lineare and the once collected O. ruahinense. Orthodontium ruahinense is not distinguished from O. lineare here, for reasons given below. The single collection (apparently by W. Bell) on which Meijer (1952, p. 44) based a N.Z. record of O. australe var. robustiusculum (Müll.Hal.) Meijer has not been available for study.