Growing solitary to scattered on the leaf sheaths deep down in clumps of Juncus conglomeratus, often found together with Mycena bulbosa (Cejp) Kühner, more occasionally also together with Hemimycena delectabilis (Peck) Singer on culms of Carex sp. Autumn. In Norway not found outside the type locality. Listed as DD in The Norwegian Redlist 2021. Lehmann & Lüderitz (2018) reported a record from Germany.

Pileus 0.7 - 2.3 mm across, conical to parabolical or convex, with or without a small papilla, more rarely hemispherical with the centre somewhat depressed, finally sometimes almost plane and the centre shallowly depressed, shallowly sulcate in some specimens, not in others, not translucent-striate, minutely pruinose, glabrescent, not lubricous when wet, white, the margin involute at first, straightening with age. Flesh very thin, white. Odour none, taste not recorded. Lamellae 0-5(-6) reaching the stipe, rarely fully developed (and then fairly broad), often only showing as low ridges and evanescent before reaching the margin of the pileus, tender, not ascending, broadly adnate to somewhat decurrent, smooth, white, the edge almost straight to concave, white. Stipe 1 - 3(-4) x 0.1 - 0.3 mm, fragile, equal or widened just below the lamellae, sometimes also broadened near the base, terete, curved, smooth, delicately pruinose all over at first, glabrescent except at the base, white, sometimes seemingly institious but actually attached to the substratum by radiating, fine, whitish fibrils which are united by a very thin film of gelatinous matter to form an irregularly shaped plaque (neither the plaque nor the fibrils being visible in dried material if the substratum has a rough surface).

Basidia 23-27 x 7 μm, clavate, 4-spored, clamped, with sterigmata c. 3.5 μm, long. Spores 9.4-10.8 x 3.6-4.5 μm, Q 2.2-3.4, Qav ~ 2.6, somewhat narrowly pip-shaped, smooth, amyloid. Cheilocystidia 20-27 x 4.5-7 μm, rather scarce, occuring mixed with the basidia, subcylindrical, subfusiform, clamped, smooth, apically gradually narrowed. Pleurocystidia absent. Lamellar trama weakly brownish vinescent in Melzer's reagent. Hyphae of the pileipellis 3.5-6 μm wide, clamped, covered with cylindrical, simple excrescences 2.5-5.5 x 1-2 μm which do not become gelatinized. Hyphae of the cortical layer of the stipe 2-2.7 μm wide, clamped, not gelatinizing, smooth or sparsely covered with cylindrical, simple excrescences 1.5-7 x 1-2 μm, terminal cells (caulocystidia) variously shaped, 20-40 x 2.5-9 μm, much branched, becoming less branched or even simple and subcylindrical farther upwards, then (just below the lamellae) gradually passing into cystidia-like, lageniform elements 22.5-35 x 6.5-9 x 3.5-4.5 μm. Hyphae of the basal plaque 2-3.5 μm wide, apparently not clamped, firm-walled, straight near the base of the stipe, embedded in a very thin film of gelatinous matter and with adhering clumps of dirt, more flexuous to kinked and thick-walled terminally, moreover mixed with some much inflated hyphae up to 13.5 μm wide.

Mycena oligophylla is easily mistaken to be a Hemimycena. It is pure white, not more than 2.3 mm across the pileus, and with poorly developed, somewhat decurrent lamellae. The spores, however, are amyloid, and the microscopic examination shows that it is a Mycena, belonging to the section Rarifoliatae Aronsen & Maas Geest.

An attempt in 2022 to obtain an ITS sequence from the holotype, unfortunately did not succeed..

Microscopic features

Collections:

VESTFOLD: TJØME, Moutmarka: 16 Oct. 1988, P. Marstad 133-88 (L, no. 986.126-094); 16 Oct. 1988, A. Aronsen M 45/88 (L, no. 988.051-005); 4 Oct. 1989, A. Aronsen A 34/89 (L, no. 986.126-085 -holotype); 4 Oct 1989, A. Aronsen A 34b/89 (L, no. 988.051-074); 4 Oct. 1989, A. Aronsen A 34c-e/89 (L, no. 988.051-194).