Description:— Pileus 20–80 mm broad, convex, convex-campanulate to plano-convex, umbonate or subumbonate, hygrophanous, translucent-striate at the margin, elsewhere dull, glabrous to slightly fibrillose, smooth, pale to dark greyish brown, paler towards margin. Lamellae adnate to emarginate, subdistant, broad, whitish then pink, edge entire to eroded, concolourous. Stipe 20–140 × 7–14 mm, central or slightly eccentric, cylindrical to compressed, broadening or tapering towards the base, fibrillose, sometimes pruinose at the apex, longitudinally striate, whitish to greyish with pale brown fibrils, and whitish fibrils at the base. Context fleshy, whitish. Odour none to farinaceous. Taste none to farinaceous.

Basidiospores 7.2–11.2 × 6.4–8.8 (–9.6) μm, Lm = 9.2, Wm = 7.8, Q = 1.0–1.3, isodiametrical to subisodiametrical, rarely heterodiametrical, 5–6 (–7) angled in side-view, colourless. Basidia 28–56 × 8–17 μm, 4-spored, clavate, colourless, clamped. Pleurocystidia absent. Cheilocystidia absent. Pileipellis a cutis to ixocutis, terminal elements 22–60 × 5–11 μm, cylindrical to cylindro-clavate, with abundant yellowish brown vacuolar and plasmatic pigment, and some yellowish encrusted pigment. Caulocystidia absent or resembling undifferentiated terminal elements 25–88 × 4–10 μm, cylindrical to cylindro-clavate, colourless, scattered or in groups at the stipe apex. Clamp-connections present in all tissues.

Habit and habitat:—Solitary, gregarious, or caespitose, on soil, in Pinus - Quercus forest and cloud forest.

Material studied:— MEXICO. Jalisco, Municipality of Tequila, volcán de Tequila, km 5–9 road to microwave transmission tower, 7 September 1986, M. Garza 216 (IBUG!); 21 September 1986, O. Rodríguez 398 (IBUG!). Municipality of Zapotlán el Grande, El Floripondio, slopes of Nevado de Colima, km 1.5 road to microwave retransmission tower, 7 September 1996, O. Rodríguez 1590 (IBUG!); 27 June 1999, C. Motolina 16 (IBUG!); 2 October 2012, D. Montañez 222 (IBUG!).

Comments:—This species keys out in Entoloma subgen. Rhodopolia sect. Rhodopolia (Largent 1974, Noordeloos 1992). It fits best with E. carolinianum Hesler, originally described from North Carolina (USA). It is characterized by the greyish brown pileus with silky-fibrillose surface and striate margin, whitish stipe, and pileipellis with both plasmatic and externally encrusted pigment (Hesler 1967, Noordeloos 1988). The Mexican material is somewhat more robust than the type collection (pileus 20–35 mm; stipe 25–45 × 3–5 mm). Entoloma carolinianum, which to our current knowledge is only known from the type locality, belongs to subgenus Rhodopolia, in which quite a few similar species have been described in the literature, both from Europe and North America. The species in this subgenus are delimited mainly on colour, type and distribution of pigment in the pileipellis, and basidiospore characteristics. When using the key to sect. Rhodopolia in Largent (1974) no compelling match was found. Entoloma atrogriseum Largent (1994) is dark, non-hygrophanous, and has smaller spores. Other similar species are E. pullum Hesler (1967), which differs because the majority of the basidiospores are isodiametrical; E. sericatum (Britzelm.) Sacc., which has a nitriclike odour; E. subradiatum (Kühner & Romagn.) M.M. Moser (1978), which has pileipellis hyphae with plasmatic pigment only; and E. minus Peck in which the pileipellis hyphae have only encrusted pigment (Noordeloos 1988, 1992). A recent study by Kokkonen (2015) on Rhodopolioid fungi in Europe shows that there is a considerable genetic variability in this group. Her attempt to translate this into a morphological or phylogenetic species concept has not been very successful. Most likely this group evolved relatively recently, leading to complexes of cryptic species that differ in a few base pairs in the ITS region, and have not yet differentiated morphologically. For this reason, we assign provisionally the epithet carolinianum to our collection.