Pileus 3-7 mm diam, hemispherical to convex, disc not depressed, margin weakly plicate; surface dull, dry to moist or gelatinous in wet conditions, glabrous to minutely pruinose; evenly pale beige. Texture membranaceous; context very thin, white.- Lamellae broadly adnate or seldom subdecurrent, distant (5-8), lamellulae absent, narrow; white to pale beige-brown; edges even, concolorous.- Stipe 4-10 x 0.3-0.5 mm, central, equal, cylindrical, curved, wiry, tough, minutely pruinose, insititious, erumpent through leaf epidermis leaving a small cupulate structure around the base; apex pale brown, fuscous to black elsewhere.- Odor and taste not distinctive.
Basidiospores 12.5-15.5 x 3.0-4.0 µm, fusoid to narrowly rhomboid, smooth, thin-walled, hyaline, inamyloid.- Basidia 24-30 x 7-8 µm, subclavate, 4-spored, clamped.- Cheilocystidia of 2 types: a) Rotalis-type cells 20-30 x 7-18 µm, clavate to ovoid, thin-walled or apically slightly thickwalled, hyaline; divergent setulae 1-2 x 1 µm, cylindrical; b) 30-48 x 6-12 µm, fusoid-capitate, capitulum up to 6 µm diam, hyaline, thin-walled.- Pleurocystidia absent.- Pileipellis hymeniform, composed of Rotalis-type cells and pileocystidia; Rotalis-type cells 20-30 x 8-20 µm, clavate to ovoid, thin-walled, apex pale reddish brown, base hyaline; divergent setulae 1-2 x 1.01.5 µm, cylindrical.- Pileocystidia 35-60 x 7-13 µm, lecythiform with elongate neck and enlarged base, hyaline, thin-walled, capitulum up to 5 µm diam, base smooth or seldom with scattered setulae.- Tissue at stipe apex dextrinoid.- Cau1ocystidia numerous, like fusoid-capitate cheilocystidia.-Clamp connections present.

Habit, habitat and distribution.- Gregarious in dense groups on rotting leaves of Pseudopanax crassifblium or P. ferox. New Zealand.

Pileus 3-7 mm, hemisphaericus vel conicus, subplicatus, glabrus vel minute pruinosus, pallide argillaceus. Lamellae late adnatae vel rariter subdecurrentes, distantes, albidae vel pallide argillaceobrunneae. Stipes 4-10 x 0.3-0.5 mm, centralis, cylindricus, minute pruinosus, insititius, apicaliter pallide brunneus sursum fuscus vel niger. Odor saporque nulli. Basidiosporae 12.5-15.5 x 3.0-4.0 µm, fusoideae vel rhomboideae, leves, hyalinae, inamyloideae. Basidia 24-30 x 7-8 µm, 4-sporigera. Cheilocystidia: a) typi Rotalis, 20-30 x 7-18 µm, clavata vel ovoidea, setulis cylindraceis 1-2 x 1 µm instructa; b) 30-48 x 6-12 µm, fusoideocapitata, capitulo -6 µm diam. instructa. Pleurocystidia nulla. Pileipellis hymeniformis, ex cellulis typi Rotalis, 20-30 x 8-20 µm, clavatis vel ovoideis, tenuitunicatis, membrana apicaliter rubrobrunnea et setulis cylindraceis, 1-2 x 1.0-1.5 µm instructa. Pileocystidia 35-60 x 7-13 µm, lecythiformia, capitulo -5 µm diam. instructa. Trama ad apicem stipitis dextrinoideum. Caulocystidia numerosa, ut cheilocystidia. Fibulae praesentes. Gregarius, ad folia putrida Pseudopanacis crassifolii. Nova Zelandia. Horak 68-152 (Holotypus, PDD).

Marasmius rhombisporus is one of three New Zealand Marasmius with pileipellis composed of Rotalis-type cells in combination with conspicuous, projecting pileocystidia. These features indicate placement in sect. Epiphylli subsect. Epiphylloidei Singer, albeit this infrageneric taxon is hardly distinguishable from some pale pigmented members of sect. Hygrometrici. Features that characterize M. rhombisporus include a small, pale beige pileus, well-developed, distant lamellae, large, narrowly rhomboid basidiospores, two types of cheilocystidia, fusoid-capitate caulocystidia, and growth on leaves of Pseudopanax. Marasmius meridionalis differs in forming smaller white pilei, a smooth hymenophore, much smaller basidiospores, and basidiome formation on leaves of Nothofagus. Marasmius rhopalostylidis differs in forming smaller ellipsoid basidiospores, fusoid-ventricose cystidia with a long non-capitate neck (never fusoid-capitate), and basidiome formation on leaves of Rhopalostylis. The European species, M. epiphylloides (Rea) Sacc. & Trott. is also similar, but differs in forming white pilei, cylindrical (not rhomboid) basidiospores, only one type of cheilocystidia, dermatocystidia that are fusoid to lageniform and only obscurely subcapitate (never capitate as in M. rhombisporus), and basidiome production is restricted to leaves and twigs of Hedera helix.

Material examind. - NEW ZEALAND: South Island, Prov. Westland, S of Ahaura, 14 III 1968, Horak 68-152 (Holotype, PDD; Isotype, SFSU, ZT).