Pileus 1-2 mm diam, hemispherical or convex when young, expanding with age to campanulate with a depressed to subumbilicate disc; surface dull, dry, glabrous; white. Texture membranaceous; context very thin.- Lame11ae absent; hymenophore smooth, vein-like or subrugose, decurrent on stipe; white.- Stipe 3-18 x 0.2-0.3 mm, central, cylindrical, equal, fragile, minutely pruinose overall, insititious; white or translucent overall.- Odor and taste not distinctive.
Basidiospores 7.0-8.0 x 4.0-4.5 µm, ellipsoid, smooth, thin-walled, hyaline, inamyloid.- Basidia 18-21 x 5-6 µm, subclavate, 4-spored, clamped.- Hymenia1 cystidia uncommon, 35-45 x 7-9 µm, lageniform-subcapitate, hyaline, thin-walled, rarely apically thick-walled.Pileipellis hymeniform, composed of Globulares-type cells, 20-30 x 1022 230m, broadly clavate to subglobose, thin-walled, hyaline; pileocystidia absent.- Pileal and 1amellar tissue inamyloid.- Stipe tissue monomitic; hyphae parallel, cylindrical, smooth, hyaline, inamyloid.- Caulocystidia rudimentary, 10-20 x 3-5 µm, as cylindrical or irregular outgrowths, hyaline, thin-walled-. Clamp connections present.

Habit, habitat and distribution.- Solitary, in groups on rotting leaves of Elaeocarpus dentatus (Elaeocarpaceae). New Zealand.

Pi1eus 1-2 mm, convexus, vel campanulatus disco subumbilicato instructus, glaber, albus. Lamellae nullae, hymenium glabrum vel subrugosum, ad stipitem decurrens, albus. Stipes 3-18 x 0.2-0.3 mm, centralis, cylindricus, minute pruinosus, insiticius, albus. Odor saporque nulli. Basidiosporae 7.0-8.0 x 4.0-4.5 µm, ellipsoideae, leves, hyalinae, inamyloideae. Basidia 18-21 x 5-6 µm, 4-spora. Cystidia hymenialia rara, 35-45 x 7-9 µm, lageniformia vel subcapitata. Pileipellis hymeniformis typi Globularium, ex cellulis late clavatis vel subglobosis instructus, 20-30 x 10-22 µm. Pileocystidia nulla. Trama pilei vel lamellarum inamyloideum. Caulocystidia 10-20 x 3-5 µm, indistincta, cylindrica, rariter diverticulis irregularibus instructa, hyalina. Fibulae praesentes. Solitarius, ad folia putrida Elaeocarpi dentatae. Nova Zelandia. Horak 69-15 (Holotypus, PDD).

Marasmius perpusillus is characterized by very small basidiomes with white, convex-subumbilicate pileus 1-2 mm diam, a smooth, decurrent hymenophore, pure white insititious stipe, relatively small basidiospores, uncommon lageniform-subcapitate hymenial cystidia, poorly developed, cylindrical caulocystidia, and the absence of pileocystidia. These features indicate placement in sect. Epiphylli, subsect. Epiphyllini. The species is similar to M. pusillissimus Desjardin & Petersen, differing by the features presented in the key. Marasmius perpusillus is not closely allied with any known temperate Northern Hemisphere taxa.

Material examined - NEW ZEALAND: South Island, Prov. Nelson, Matakitaki River Valley, Murchison, 24 I 1969, Horak 69-15 (Holotype, PDD; Isotype, SFSU, ZT).