Pileus 2-3 mm diam, hemispherical to convex, laterally attached to substrate, with a single transecting shallow groove over lamella; surface dull, dry, glabrous to minutely pruinbse; evenly off white to beige. Texture membranaceous; context very thin, white.- Lamella only one, adnate at point of lateral attachment, narrow to moderately broad, white; lamellulae absent-. Stipe absent.- Odor and taste not distinctive.
Basidiospores 9.0-12.5 x 4.5-5.0 µm, ellipsoid to subfusoid, smooth, thin-walled, hyaline, inamyloid.- Basidia 30-36 x 7-9 µm, (1-, 2-) 4-spored, cylindrical to subclavate, clamped.- Cheilocystidia scattered, Rotalis-type cells similar to those on the pileipellis.- Pleurocystidia absent.Pileipellis hymeniform, composed of Rotalis-type cells; main body (15-) 20--40 x 12-30 µm, broadly clavate to sphaeropedunculate, thin-walled at the base, thick-walled at the apex, pale yellowish brown; divergent setulae 1-2 x 12 µm, cylindrical.- Tissues inamyloid, non-gelatinous.- Clamp connections present.

Habit, habitat and distribution.- Solitary, in groups on rotting leaves of Beilschmiedia tawa (Lauraceae). New Zealand.

Pileus 2-3 mm, hemisphaericus vel convexus, lateraliter ad substratum affixus, glaber vel minute pruinosus, subalbidus vel argillaceus. Lamella una sola. Stipes nullus. Odor saporque nulli. Basidiosporae 9-12.5 x 4.5-5.0 µm, ellipsoideae vel subfusoideae, leves, hyalinae, inamyloideae. Basidia 30-36 x 7-9 µm, (1-, 2-) 4-spora. Cheilocystidia typi Rotalis, rara, cellulis pileipellidis similibus. Pleurocystidia nulla. Pileipellis hymeniformis typi Rotalis, ex cellulis, late clavatis vel sphaeopedunculatis, 20-40 x 12-30 µm, setulis cylindricis, 1-2 x 12 µm instructis. Trama inamyloideum, haud gelatinosum. Fibulae praesentes. Solitarius, ad folia putrida Beilschmidiae tawae. Nova Zelandia. Horak 1112 (Holotypus, PDD).

Marasmius unilamellatus is unusual in the genus in forming basidiomes with a single lamella extending from the sessile pileus margin to the point of attachment on the substratum. An undescribed species from the Hawaiian Islands is the only other Marasmius species that shares this feature, although the Hawaiian species forms a well-developed lateral stipe and grows on monocotyledous leaves (Desjardin, unpubl. data). It is difficult to determine whether M. unilamellatus belongs in sect. Marasmius or sect. Hygrometrici, showing anatomical features consistent with both infrageneric taxa. In the field, M. unilamellatus may appear similar to several "2-holer" Favolaschia species; however, Favolaschia lacks a pileipellis composed of a hymeniform layer of Rotalis-type cells.

Material examined- NEW ZEALAND: North Island, Prov. North of Auckland, Waipoua, Kauri Reserve, 22 VI 1981, Horak ZT 1112 (Holotype, PDD; Isotype, SFSU, ZT).