Gregarious among grass in open field and in moss under Juniper. Autumn. Known from Norway, Finland and Sweden.

Pileus 10-20(-25) mm across, at first acutely conical, flattening with age and becoming more or less irregularly campanulate, finally plano-convex, more or less umbonate, or somewhat depressed at the centre, little to shallowly sulcate, translucent-striate, pruinose, glabrescent, viscid, covered with a gelatinous separable pellicle, blackish grey, blackish brown to dark grey-brown, paler when dry, often darker at the centre and the margin. Context thin. Odour indistinct or somewhat farinaceous. Lamellae 20-32 reaching the stipe, tender, arcuate, c. 1 mm broad, more or less rugulose with age, decurrent with a tooth, pale to dark grey, with a separable paler edge. Stipe 30-65 x 1.5-3.5 mm, hollow, egual or somewhat broadened above, curved below, terete to somewhat depressed, smooth, pruinose, glabrescent except for the apex, viscid, greyish, usually darker below, covered with few fibrils at the base.

Basidia 30-40 x 7-9 µm, slender-clavate, 4-spored, with sterigmata up to 6.5 µm long. Spores 9.2-10.3 x 4.7-5.4 µm, Q = 1.8-1.9, pip-shaped, smooth, weakly amyloid. Cheilocystidia 18-32 x 5.5-7 µm, forming a sterile band, clavate, embedded in gelatinous matter, apically covered with comparatively few, unevenly spaced, coarse, occasionally curved, cylindrical to clavate excrescences 2.5-14.5 x 1.5-5.5 µm. Pleurocystidia absent. Lamellar trama dextrinoid, brownish vinescent in Melzer's reagent. Pileipellis an ixocutis of much branched, smooth, hyphae 1.5-2.5 µm wide, the terminal cells 1-1.5 µm wide, apically diverticulate and much branched, with the excrescences 0.9-3.5 x 0.9 µm. Hypoderm made up of parallell, inflated hyphae up to 20 µm wide. Hyphae of the cortical layer of the stipe 1.5-2.5 µm wide, embedded in gelatinous matter, smooth, terminal cells not observed. Clamp connections present at all tissues.

Mycena agrestis is a species of section Fuliginellae (A.H. Sm. ex Singer) Maas Geest., of which only seven species are known, four in North America and three in Europe. Four species have, like M. agrestis, arcuate lamellae. M. austinii (Peck) Kühner can be distinguished because of its white pileus, presence of a basal disc and smaller spores. M. mackinawensis A. H. Sm. is different because of smaller spores, differently shaped terminal cells of the hyphae of the pileipellis, and occurrence on coniferous branches. M. vulgaris (Pers.) P. Kumm. differs in having smaller spores, differently shaped cheilocystidia, and occurrence with conifers. M. geesterani Heykoop, Esteve-Raventós & Moreno, recently described from Spain (Heykoop et al. 1992), is supposed to have much smaller spores that are strongly amyloid, differently shaped cheilocystidia and occurrence with conifers.

Because of the viscid appearance of the pileus and stipe M. agrestis could be taken for a member of sect. Hygrocyboideae. M. agrestis, however, never shows any yellow colours, which are so conspicuous in most varieties of Mycena epipterygia, and in the latter the lamellae are ascending and adnate (although decurrent with a tooth), never arcuate. The two varieties of M. epipterygia lacking yellow shades, var. fuscopurpurea and var. pelliculosa can in the field be separated from M. agrestis with arcuate lamellae. Besides, they can be distinguished on account of the microscopic features. In M. epipterygia the hyphae of the pileipellis are branched, anastomosing, covered with simple to furcated or branched excrescences, often forming dense masses, not smooth as in M. agrestis; the hyphae of the cortical layer of the stipe are smooth to sparsely covered with excrescences (smooth in M. agrestis); terminal cells abundant, coarsely diverticulate (presumably absent in M. agrestis).

In 2009 Mycena agrestis was recorded for the first time in Finland (von Bonsdorff & Aronsen 2011), and 2011 it was found in Skåne, Sweden (L. Örstadius, pers. comm.).

A recent ITS sequence of the holotype of M. agrestis showed 99.56% similarity to M. epipterygia and strongly indicates that it should be considered as a synonym of the latter.

Norwegian collections:

VESTFOLD:
TJØME, Moutmarka 7 Nov. 1992. A. Aronsen A 53/92 (L, no. 993.342-029); 9 Oct. 1993. A. Aronsen A 66/93 (holotype; L, no. 993.342-087); 27 Oct. 1996, A. Aronsen A 34/96; 1 Nov. 1998, A. Aronsen A 4/98 (O); 2 Nov. 1999, A. Aronsen A1/99 (O); 15 Oct. 2006, A. Aronsen A13/06.

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