Gregarious under Alnus incana (and more rarely Picea abies). A few Finnish records and one record from Norway.

Pileus 10-50 mm across, at first convex, remaining so or becoming irregularly plane with an undulating partly elevated margin, glabrous, hygrophanous, weakly translucent-striate when moist, sordid lilac when fresh, more brownish in age; surface dry, slightly rugose. Lamellae 28 to more than 30 reaching the stipe, ascending, adnate to broadly adnate or almost decurrent, becoming dorsally intervenose, sordid lilac, densely punctate by minute, dark purplish brown dots (pleurocystidia), and with entirely dark violet edge. Stipe 30-70 x 3-10 mm, occasionally somewhat rooting, widened downwards but basally narrower, hollow, straight, curved below, terete, covered with conspicuous, very small dark granules at the apex, dry, pale sordid lilac, yellowish at the base. Odour distinctly of radish. Taste mild, of radish. All parts of the basidiocarp often blackening when drying.

Basidia 27-30 x 6.5-8 µm, slender-clavate, 4-spored. Spores (6-)7.5-9 x (3.5-)4-5 µm, Q = 1.4-2.3, Qav ~ 1.8, pip-shaped or somewhat cylindrical, smooth, weakly amyloid. Cheilocystidia 43-88 x 7-17 µm, occuring mixed with basidia, strongly protruding, fusiform, with long, narrow necks, with purplish brown contents. Pleurocystidia numerous, similar, but often shorter, with purplish brown contents. Hyphae of the pileipellis up to 3.5 µm wide, smooth. Hyphae of the cortical layer of the stipe up to 2.5 µm wide, smooth, the terminal cells cylindrical, curved outwards. Clamp connections present at all tissues.

Microphotos of cheilocystidia and spores.

The macroscopic description has been taken from Harmaja's and Maas Geesteranus' descriptions complemented by my own observations on the dried material. The microscopic details are based on examination of five Finnish collections.

Harmaja (1985: 44) described the until then unknown species M. lammiensis from Finland under Alnus incana (and more rarely Picea abies). It shares many features with M. pelianthina, like the violaceous tints, the raphanoid smell, and the dark violet lamellar edge. Microscopically the two species are quite similar too, the only distinct difference being the spore size. In M. pelianthina the spores rarely are more than 4 µm broad, while in M. lammiensis they are almost always broader than 4 µm. The shape of the spores may be somewhat different too. In M. pelianthina the spores are somewhat narrowly pip-shaped, while in M. lammiensis they are pip-shaped to somewhat cylindrical. This is, however, hardly a distinct specific character. Harmaja claimed that the cystidia of M. lammiensis were narrower than the counterparts in M. pelianthina, and that they had longer and narrower necks. Although the outer measurements do not differ much, my impression is that Harmaja is right on this point. The cheilocystidia generally seem to be narrower and with narrower and longer necks in M. lammiensis. Another difference is that the cheilocystidia are much more protruding in M. lammiensis than the counterparts in M. pelianthina.

It is not clear to me whether the colour of the pileus is different between the two species. In Mycena pelianthina the colour is date brown to pale brown with purplish to violaceous tinges, drying to pale ochraceous or beige, with or without a pinkish shade. The descriptions of M. lammiensis suggest a more dingy lilac colour, which is turning more brownish with age. The blackening process when drying seems to be similar in both species.

Harmaja (1985) and Maas Geesteranus (1992: 405) discussed the very slight differences between M. lammiensis and the American species M. rutilantiformis, and concluded somewhat hesitantly that M. lammiensis is a species in its own right. The recognition of M. lammiensis as a distinct species different from M. pelianthina was recently supported by phylogenetic analyses (Harder et al. 2010) also showing that the average spore width is a valid criterion to discriminate between the two species.

Mycena lammiensis is known from 15 collections in Finland and is red listed as Near Threatened (NT) (Finnish Biodiversity Information Facility 2023). An interesting question is if the Norwegian collections of M. pelianthina, collected under Alnus, from central and Northern parts of the country actually would prove to represent M. lammiensis, as suggested by Harmaja (2002: 25). Examination of three collections from the herbarium in Oslo, and five collections from Tromsø, however, showed without doubt that they are M. pelianthina. This has been cofirmed by sequences in the Norwegian Barcoding Project (NorBOL). In 2021, however, the first record of M. lammiensis was registered under Alnus at Dovre, Innlandet. The collection had spores matching M. lammiensis and matched a Finnish sequence in GenBank 100%.

Mycena lammiensis belongs to the section Calodontes on account of the large size, the purplish or violaceous tints, and the raphanoid odour. Within this section it is placed in subsection Marginatae together with M. pelianthina and M. rutilantiformis because of the intensely coloured lamellar edge. The latter has only been recorded from the United States. Harmaja (2002) transferred Mycena lammiensis to the genus Prunulus, a point of view that has not been followed at this web site.

I will express my thanks to the authorities of the herbarium in Helsinki for the loan of the holotype of M. lammiensis and an additional collection from Turku (Leg. S. Huhtinen, 30 Aug. 1985), and to J. Vauras, Turku, for providing me with copies of photos and an additional collection.

Collections examined:

FINLAND:
Etelä-Häme: Lammi, Hauhiala, S shore of Lamminjärvi 5 Sept. 1978, Anne Sairanen (Holotype, H)
Sb, Vehmersalmi, Puutosmäki, Pitkälahti, grid 6956:541, by the old limestone quarries, thicket of Alnus, with Daphne, Populus tremula 30 Aug. 1985, S. Huhtinen 85/85 (TUR)
PS. Vehmersalmi, (Soisalo), Puutosmäki, grid 27° E: 69568:5413, 12 Aug. 1992, Pekka Heinonen 60-92 (TUR).
Keski-Pohjanmaa, Kalajoki commune, Rahja-Roukala, grid 27° E: 7125:341, damp, herb-rich forest with mainly Alnus incana and scattered Prunus padus, Picea abies, and Betula, on litter 11. Sept. 2004, Katri Kokkonen 271/04 (TUR).
Koillismaa, Kuusamo, Oulanka National Park, Liikasenvaara, grid 27° E: 7362:612, moist brookside forest with Betula, and Alnus incana, under Alnus incana, 24 Aug. 2005, leg. Esteri Ohenoja, Jukka Vauras 23266F (TUR).
Outer Ostrobothnia (PeP), Rovaniemi, Omalamminoja, 05.09.2013, leg. A. Aronsen & T. von Bonsdorff (herb. Aronsen).

NORWAY:
Innlandet, Dovre, Joris delta, 12. Aug. 2021, 'I fukta oreskog'. Leg. Perry Larsen, Tor Erik Brandrud, Siri Khalsa (O-F-258897).