Full name: Hebeloma gigaspermum Gröger & Zschiesch. (1981)
Published: Z. Mykol. 47 (2): 201 (1981)
Taxonomic status: Heterotypic synonym of H. nauseosum
Types: GERMANY: Thuringia, Siebleber Teich, Gotha (approx. 50.937°N, 10.7632°E, alt. approx. 280 m a.s.l.) under Salix sp., 28 Sep. 1978, F. Gröger (Holotype. held at herbarium JE, HJB1000148).

Homotypic synonyms: Hebeloma groegeri Bon (2002)

Original diagnosis: Cortina nulla. Pileo 1,8-5 cm lato, obtuse usque plane conico, late campanulato et obtuse umbonato, margine tamen usque ad senectutem inflexo vel (praecipue in iuventute) supra lamellas inducto et nonnumquam crenulato, in ambitu saepe irregulari, saepe parum excentrico, perfecte glabro in statu humescente glutinosiore, mox sicco, ad marginem interdum subtomentoso, rarissime (et maxime in margine inflexa) albido, ad marginem ochraceo, fulvo-ochraceo vel fumoso, alibi luteo-ochraceo, luteofusco vel sordide fusco-ochraceo, in centro etiam rufescenti-ochraceo. Lamellis fere distantibus, haud nimis latis (in adulto fungi in media parte lamellarum 5 mm latis sub pileo 2-3 mm crasso), parum undulatis usque grosse crenato-serratis, minutissime albide vel pallide ciliatis, angustissime adnexis vel profunde sinuatis, primo fusco-ochraceis vel instar coffeae cum lacte mixtae coloratis, deinde fumosis, laete olivaceis (non adeo obscuris ut in H. tomentoso vel H. latifolio), distincte olivascentibus (si ad superficiem spectes), nonnulis transverse venosis et inter se conjunctis. Stipite 3-6,5 x 0,3-0,7 (1) cm, pallido, deinde (primo deorsum) fuscescente, in summa parte subfarinaceo vel subpruinoso (nonnunquam etiam fere glabro), alibi glabro et innato-fibrilloso, +/- aequali, ad basim interdum leviter incrassato et saepius fere fusiformiter in substratu radicante, in iuventute deorsum etiam subclaviformi, raro breviter acuminato tantum, farcto. Carne amarescente vel distincte amara, albida, partim tamen in statu humescente hyalina, colore pelliculae pilei leviter invadente, in basi stipitis nigrescente. Odore fragrante, saponaceo, Hebelomati sacchariolenti simili. Basidiis 4-sporis, 39,5-46 x 10-12,2 μm. Sporis longius citriformibus vel amygdaliformibus, magnitudine valde inaequali, (11) 12-15,5 (17-19) x 6,5-7,8(8,5) μm, perspicue verrucosis, luteo-fuscis, partim leviter papillosis, cum duplici pariete, exosporio distincte levato. Cheilocystidiis fere 35-40 μm marginem lamellae superantibus, in iuventute subclaviformibus usque valde capitatis, deinde fere cylindraceis usque anguste claviformibus, 57-85 x (6) 7,5-12 (14-15) μm. Epicutis submucosa, hyphis laxe contextis, 2,5-4,5 μm crassis, nonnuliis crassiolibus intermixtis, constante.

English translation: Cortina absent. Pileus 1.8–5 cm broad, bluntly and plano-convex, broadly campanulate and bluntly umbonate, margin however, inflexed up to old age or (especially when young) exceeding the lamellae and never crenulate, in outline often irregular, often slightly eccentric, perfectly glabrous in moist state somewhat glutinous, soon dry, sometimes subtomentose at margin, very rarely whitish (and mostly at inflexed margin), at margin ochre, reddish ochre or smoke-brown, elsewhere yellow-ochre, yellow-brown or sordid reddish ochre, in centre reddish ochre. Lamellae more or less distant, not very broad (in mature basidiomes in middle part 5 mm broad, 2–3 mm wide under pileus), slightly undulate and coarsely crenulate-toothed, minutely white or pale ciliate, broadly adnexed or deeply sinuate, brown-ochre at first or milky coffee, then smoky-brown, bright olivaceous but not so dark as in H. tomentosum or H. latifolium), distinctly turning olivaceous if you look closely at the surface, sometimes transvenose or interconnected. Stipe 3–6.5 × 0.3–0.7 (1) mm, pallid, then (at first at base) turning brown, farinaceous to subpruinose in upper part, sometimes almost glabrous, white glabrous or innately fibrillose, more or less equal, sometimes slightly broadened at base, often almost fusiform, rooting in substrate, when young subclavate at base, rarely acuminate only, stuffed. Context bitterish to distinctly bitter, white, partly hyaline in humid state, the colour of the pileal surface slightly invading in the context, blackening in base of stipe. Odour fragrant, soapy, and similar to that of H. sacchariolens. Basidia four-spored, 39.5–46 × 10–12.5 μm. Spores long citriform or amygdaloid, large and rather unequal, (11) 12–15.5 (17–19) × 6.5–7.8 (8.5) μm, clearly verrucose, yellow-reddish brown, partly slightly papillose, with double wall, exosporium distinctly loosening. Cheilocystidia exceeding 35–40 μm the margin of the lamellae, subclaviform when young and strongly capitate, then more cylindrical to narrowly claviform, 57–85 × (6) 7.5–12 (14–15) μm. Epicutis subgelatinous, made up of intermixed hyphae, 2.5–4.5 μm wide, interspersed with wide hyphae.

Commentary: Unfortunately, we have been unable to amplify DNA and generate a sequence. However, we have been able to study the type material. The spores measure, on average, 15.2 × 8.6 μm and given the description of the pileus this is: = Hebeloma nauseosum (Cooke) Sacc.