Basidiomata scattered on the ground under Juniperus communis. Autumn. Only the holotype is known from Norway. Robich (2003: 238- 241) reported it from Italy. He also provided a colour photo.

Pileus (dried) up to 8 mm across, convex, without umbo, sulcate, pruinose, grey-brown. Flesh thin. Odour not recorded. Lamellae 16 reaching the stipe, tender, ascending, c. 1 mm broad, somewhat ventricose, adnate, decurrent with a short tooth, grey, the edge convex, pallid. Stipe (dried) 35-45 x 1 mm, hollow, fragile, equal, terete, apically densely pruinose, glabrous farther below, grey-brown, the base densely covered with long, coarse, white fibrils.

Basidia 22.5-27 x 7-9 µm, clavate, 4-spored, clamped, with sterigmata 5.5-6.5 µm long. Spores 7.6-8.2 x 4.9-5.5 µm,pip-shaped, smooth, amyloid. Cheilocystidia 27-50 x 9-19 x 0-4.5 µm, fusiform, sublageniform (a few without neck and more ellipsoid), clamped, smooth. Pleurocystidia not numerous, mostly without neck, ellipsoid. Lamellar trama brownish vinescent in Melzer's reagent. Hyphae of the pileipellis 1.8-2.5 µm wide, clamped, covered with simple or furcate to somewhat branched, cylindrical excrescences 1.8-15 x 1.5-2 µm. Hyphae of the cortical layer of the stipe 1.8-2.5 µm wide, clamped, sparsely covered with simple to furcate, curved to more or less coiled excrescences 2-9 x 1-2 µm, the terminal cells 4.5-8 µm broad, variously shaped and generally coarsely diverticulate.

The description has been taken entirely from Maas Geesteranus (1993: 337-338).

Mycena aronsenii belongs to sect. Fragilipedes where it comes close to M. leptocephala. The latter differs in having smooth hyphae of the cortical layer of the stipe, generally with much inflated caulocystidia, larger spores, and the hyphae of the pileipellis tending to become somewhat gelatinized. Following the key in Maas Geesteranus (1992) M. aronsenii comes close to M. abramsii (Murill) Murill and M. aetites (Fr.) Quél. The first species has, among other things, more lamellae reaching the stipe (19-27), larger spores, and differently shaped excrescences of the hyphae of the stipe. The latter has, among several differences, differently shaped cheilocystidia, larger spores, and different shape of the excrescences of the hyphae of the stipe.

M. aronsenii belongs to a very difficult complex within sect. Fragilipedes, and the type was poorly annotated macroscopally. A recent attempt (2022) to obtain an ITS sequence from the holotype unfortunately was unsuccessfull. For the time being I reluctantly accept it as a separate species.

VESTFOLD, TJØME, Hvasser, Sønstegård, gregarious under Juniperus, 4 Nov. 1989, A. Aronsen A 67/89 (No. 989.027-053; L).