NEW ZEALAND: North Island, Northland, Hokianga Co., Trounson Kauri Park, 13.V.83, G. Samuels (R. H. Petersen no. 55648) [Holotype, TENN no. 44415]; Waipoua Kauri Reserve, 12.V.83, R. H. Petersen no. 55638 (TENN no. 44419].

Basidiomata marcescent. Pileus 0.2-0.5 mm diam, convex, deeply sulcate; surface granulose, white overall. Lamellae adnate or slightly adnexed, with or without a poorly developed collarium, remote (3-5 complete lamellae), broad, white; edges granulose; lamellulae absent. Stipe 0.5-1 x 0.1 mm, central, curved due to geotropic hymenophore, filiform, terete, equal, white overall; surface glabrous above, basal region with a few mycelial hairs, subinsititious, often with a socle-like base surrounded by a small ring of radiating hairs. Rhizomorphs absent. Odour and taste not recorded.
Basidiospores 7.8-9.6 x 4.0-4.8 µm [x = 8.6 x 4.4 µm, s = 0.5 x 0.3, E = 1.7-2.1, Q = 1.9, sQ = 0.1, n = 20], ellipsoid or amygdaliform, hyaline, inamyloid, smooth. Basidia 16-20 x 8-10.5 µm, tetrasporic, broadly clavate. Basidioles broadly clavate. Pleurocystidia absent. Cheilocystidia common nearest pileus margin, scarce nearest stipe, similar to the pileipellis elements. Pileipellis hymeniform, of Rotalis-type elements, 12-20 x 8-16 µm, broadly clavate or sphaeropedunculate, hyaline, inamyloid, thin-walled; diverticula 1.0-2.5 x 0.5-1.5 µm, cylindric-obtuse, hyaline, thick-walled. Tramal hyphae interwoven in pileus, subparallel in lamellae, 2.5-4.0 µm diam, non-inflated, non-gelatinous, hyaline, inamyloid or weakly dextrinoid, thin-walled, clamp connections common; refractive oleiferous hyphae occasional. Stipe tissue monomitic; cortical and medullary hyphae similar, parallel, 2.0-7.2 µm diam, hyaline throughout, strongly dextrinoid, thin-walled, smooth; clamp connections absent. Stipe vesture absent near apex, consisting of a few erect, filiform, mycelial "hairs" nearest the base; caulocystidia absent. Scattered on senescent leaves of Gahnia sp. [Cyperales].

Pileus 0.2-0.5 mm latus, convexus, sulcatus, albus. Lamellae adnatae, collariatae, remotae, albae. Stipes 0.5-1 x 0.1 mm, centralis, falcatus, subinsititius, albus. Rhizomorphae nullae. Basidiosporae 7.8-9.6 x 4.0-4,8 µm, ellipsoideae vel amygdaliformes, laeves, hyalinae, inamyloideae. Basidia tetraspora. Pleurocystidia nulla. Cheilocystidia elementis pileipellis similia. Pileipellis hymeniformis ex elementis M. rotali similibus; diverticuli hyalini. Trama ex hyphis inamyloideis vel dextrinoideis, raro Fibulatis. Stipes ex hyphis dextrinoideis, defibulatis. Caulocystidia nulla. In foliis Gahniae.

Diagnostic features of M. rosulatus include: a) minute stature, with pilei reaching 0.5 mm diameter and stipes up to 1 mm long; b) reduced number of well-developed lamellae (3-5 present, no lamellulae) and a poorly developed (or absent) collarium; c) complete lack of pigments throughout the basidiomata; d) clamp connections on tramal hyphae and hymenial elements but absent in stipe tissue; e) strongly dextrinoid stipe tissue; f) subinsititious stipe with radiating basal mycelial hairs; and g) growth on senescent leaves of Gahnia.
We were unable to demonstrate the presence of a collarium in the holotype specimen, but a second, conspecific collection contains several basidiomata, which readily show a thin layer of tissue connecting the lamellae and sheathing the stipe apex. Presence of a collarium, albeit very weakly developed, in combination with dextrinoid stipe tissue and Rotalis-type pileipellis elements dictates placement in sect. Marasmius subsect. Marasmius [= subsect. Pararotulae Singer (1976)]. It must be noted, however, that a subinsititious stipe and a complete absence of pigments exhibited by M. rosulatus are characters unusual in sect. Marasmius. Within subsect. Marasmius, M. rosulatus is most phenetically similar to M. pararotula Sing., described from Bolivia. Both species form minute, white pilei with few, broad lamellae. Marasmius pararotula differs in forming a deep umber, longer (5-12 mm) and strictly insititious stipe, more lamellae (5-8), broader spores (4.8-5.5 µm diam.), and in growing on leaves of dicotyledonous plants (Fide Singer 1965).

Holotypus: TENN no. 44415.