Pileus to 3 mm diam., to 4 mm high, thimble-like or bell-shaped with a lateral slit, hanging like a street lamp, white, thin-fragile (almost hyaline), smooth. Lamellae none, hymenophore smooth, white. Stipe to 6 X 0.2 mm, central to eccentric, emerging through the lateral slit, cylindrical, whitish to transparent-hyaline overall, very fragile, minutely pruinose, dry, solid, solitary in groups, byssus or basal disc absent. Odour absent. Basidiospores 7-8.5 X 3-3.5 µm, slender elliptic to subcylindric, smooth, hyaline, inamyloid. Basidia 15-20 X 5-6 µm, tetrasporic, cylindric to subclavate, sterigmata up to 4 mm long, clamped. Hymenial cystidia absent. Pileipellis a cutis of cylindric to subfusiform hyphae (3-) 5-16 µm diam., non gelatinous, thin-walled, smooth (except with few, scattered prong-like projections up to 7 µm long), hyaline, inamyloid, clamped. Pilocystidia absent. Oleiferous hyphae absent. Stipe tissue monomitic. Stipe conical hyphae 3-15 µm, cylindric, parallel, non-gelatinous, thick-walled (up to 2 µm), hyaline, inamyloid, clamped. Caulocystidia scattered, 20-40 µm long, awl-shaped or subfusiform with tapering neck, hyaline, walls up to 1 µm thick.

On rhachis of Dicksonia sp. (tree-fern). Known from North Island of New Zealand.

Pileus usque ad 3 mm latus, usque ad 4 mm altus, cylindricus vel campanulatus, albus, fragilis, glaber. Lamellae nullae, hymenium glabrum, album. Stipes usque ad 6 X 0.2 mm, centralis, eccentricus, vel lateraliter affixus, cylindricus, albidus, transparenter hyalinus, fragillimus, minute pruinosus, siccus. Inodorus. Basidiosporae 7-8.5 X 3-3.5 µm, ellipticae vel subcylindricae, leves, hyalinae, inamyloideae. Basidia 15-20 X 5-6 µm, tetrasporica, fibulata. Cystidia hymeniales nulla. Pileipellis cutis ex hyphis cylindraceis vel subfusiformibus cutem formantibus, (3-) 5-16 µm latis, baud gelatinosis, membrana hyalina, tenuitunicata glabraque instructa. Pilocystidia nulla. Caulocystidia rara, 20-40 µm longa, acuto-fusiformia. Ad folia putrida Dicksoniae. Novazelandia. PDD, holotypus.

Hemimycena reducta is characterised by forming very small, bell-shaped basidiomes with a smooth hymenophore and a central to eccentric stipe that emerges through a slit in the pileus. In addition, the pileipellis is formed from nearly smooth hyphae (not a Rameales-structure), hymenial cystidia and pilocystidia are lacking, and caulocystidia are awl-shaped and often setoid. In the field, basidiomes are suggestive of those of Rimbachia Patouillard (1881) and Calyptella Quelet (1886), however, a true stipe is not formed by members of either of these genera. The habit of H. reducta is also suggestive of the genus Cymatella Patouillard (1889), however, in the latter genus basidiomes develop pigmented stipes, and have fusoid basidioles and Rameales-type pileipelli.
Hemimycena reducta differs from other Hemimycena species with reduced hymenophore [such as H. minutissima Desjardin (1991), H. hirsuta (Tode) Singer (1986), Helotium cyphelloides Redhead (1982), Helotium nebulophilum Redhead (1982), Helotium circulare (Singer) Redhead (1984)] by the combination of lacking pilocystidia, lacking numerous diverticula on pileipellis hyphae, and having tetrasporic basidia.

Holotype
New Zealand, North Auckland, Omahuta, 11.v. 1981, Horak (PDD; ZT 543, isotype).