Calomnion Hook.f. & Wilson

Plants shiny, small, olive-green to light green, with an extensive and persistent protonema, usually on tree-fern caudices. Stems simple, beset at base with papillose brown rhizoids, in cross-section with 2–3 layers of thick-walled cells surrounding several layers of larger thin-walled cortical cells and a small central strand (often collapsed in older stems). Leaves tristichous, dimorphic, often caducous; lateral leaves in two ranks, ovate to elliptic-lanceolate, entire or denticulate, reduced in size at stem base; ventral leaves in a single rank, smaller, and usually broadly elliptic; upper laminal cells polygonal, ± isodiametric, smooth, flat to slightly bulging; basal laminal cells and alar cells not differentiated. Costa single, robust, subpercurrent, percurrent, or excurrent.

Dioicous. Perichaetia terminal and with leaves much longer than the vegetative, erect and sometimes sheathing. Perigonia terminal. Setae erect or ± flexuose, elongate; capsules cylindric, erect, gymnostomous, ellipsoid or cylindric, smooth or wrinkled when dry; stomata restricted to capsule base; annulus well-differentiated or absent; peristome absent. Spores papillose. Calyptra cucullate, ± scabrous above.

The number of species in Calomnion requires reconsideration in a monographic context. Vitt (1995) considered the genus to include nine species distributed in the South Pacific, from Malesia south to Tasmania and N.Z., and east to Tahiti, and attempted to correlate speciation and dispersal with tectonic events in the South Pacific. However, one of his species (C. brownseyi) is rejected here for the reasons stated below. The morphological features used to differentiate some species (e.g., what Vitt termed C. brownseyi and C. melanesicum) likewise need re-evaluation. Only one species of Calomnion is accepted for N.Z.

Calomnion laetum is an illegitimate name (Article 52, McNeill et al. 2011) because the epithet of an earlier legitimate name is cited in synonymy in its protologue, as noted by Vitt (1995). Therefore, in accordance with Article 7.5, the type of the genus is C. complanatum (Hook.f. & Wilson) Lindb. (Gymnostomum complanatum Hook.f. & Wilson).

The relationships of Calomnion have been problematic since its description, largely because of its lack of a peristome. Brotherus (1924) placed it in its own family in the general relationship of the Rhizogoniaceae and the Mitteniaceae. Dixon (1932) retained the Calomniaceae but considered it to be allied to nematodontous groups and placed it close to the Georgiaceae (=Tetraphidaceae) and Schistostegaceae. Sainsbury (1955) seemed to have accepted Brotherus’ earlier view. Vitt (1995) reached no firm conclusion as to the familial relationships of the Calomnion. He cited reasons for following its "more traditional Bryalean placement near Rhizogonium" but also noted that its "capsules, seta, and protonema are at least superficially similar to those of the Tetraphidaceae." In their more recent and influential analysis using multiple molecular markers, Bell et al. (2007) found Calomnion to be nested within the Rhizogoniaceae. Goffinet et al. (2009) also placed Calomnion in the Rhizogoniaceae, and this placement is accepted here.

There is some confusion concerning the interpretation of the position (i.e. "ventral" vs "dorsal") of the rank of reduced leaves in Calomnion. Their position is interpreted here as fundamentally similar to those in such genera as Hypopterygium and can justifiably be labelled "ventral" as done by Vitt (1995).

The orientation in C. complanatum of the reduced rank of leaves in a skyward position is likely an orientation for increased light gathering in a deeply shaded forest habitat. Developmental torsion of the stems is probably involved, but confirmation of this would involve detailed morphogenetic study.