Ardeuma R.H.Zander & Hedd.

The following generic description is modified from Zander (1993) (as Hymenostylium), Smith (2004) (as Hymenostylium), and Zander & Hedderson (2016).

Plants forming turves or cushions, commonly on calcareous rock. Stems often branching, in cross-section rounded-pentagonal to triangular, with central strand absent, sclerodermis usually present (not in N.Z. species), and hyalodermis usually absent (weakly developed in N.Z. species). Leaves often distant on stem, ligulate to lanceolate or linear-lanceolate, usually acute; margins plane to broadly recurved, entire or papillose, rarely serrulate near leaf apex; upper laminal cells clear in outline, usually heterogeneous in size and shape, thin- to firm-walled, often porose (not in N.Z. species), pluripapillose with simple papillae; marginal cells weakly differentiated; lower laminal cells differentiated, rectangular, thin- to firm-walled. Costa often stout, usually excurrent but sometimes ending up to 5 cells below leaf apex (in N.Z. species), superficial adaxial cells usually elongate, in cross-section usually with 2 stereid bands (not well defined in N.Z. material). Axillary hairs usually c. 8-celled. Laminal KOH colour reaction yellow.

Dioicous. Perichaetia terminal, with inner perichaetial leaves weakly differentiated. Perigonia terminal, bulbiform. Setae elongate. Capsules ovoid to short-rectangular. Operculum obliquely rostrate. Peristome lacking. Spores 12–14 µm, weakly papillose.

The genus Hymenostylium was erected by Bridel (1827) to accommodate a Nepalese taxon Gymnostomum xanthocarpum (Hook.) Brid. An evaluation of Hymenostylium by Zander & Hedderson (2016)​​​​​​​ resulted in the transfer of almost all other species to a new genus, Ardeuma R.H.Zander & Hedd. Salient distinctions of this new genus include the lack of a stem central strand, leaves widest at the base, and laminal papillae small and multiple over each lumen.

Widespread in the world, comprising some 14 species.