Marasmius horqinensis T. Bau & W. N. Hou 2026, sp. nov.

Marasmius horqinensis T. Bau & W. N. Hou sp. nov. Figs 4 D, 4 E, 7 Diagnosis. It is similar to Ma. elaeocephalus but differs in having adnate, distant (L = 8–10, I = 1–2) lamellae and Rotalis - type cheilocystidia. Etymology. “horqinensis” refers to the type locality, Horqin Left Wing Rear Banner, Tongliao City, Inner Mongolia Autonomous Region, China. Description. Basidiomata small. Pileus 0.2–0.5 cm diam, initially plano-convex, becoming nearly plane at maturity; surface tough, smooth, cream-yellow (RAL 1002) to ochre (RAL 1024), with radial grooves, slightly depressed at center, often wrinkled in the depression; margin entire, straight. Context cream (RAL 9001), very thin; odor indistinct. Lamellae adnate, distant, unequal, very narrow, 0.5–1 mm wide, L = 8–10, I = 1–2, cream (RAL 9001). Stipe 0.3–1 cm long, 0.5–1 mm thick, cylindrical, equal or slightly thicker at base, cream (RAL 9001), smooth, hollow, tough; base directly attached to habitat. Basidiospores (8) 8–12 (13) × (3) 3.4–4.8 (5.2) µm, Q = (2.05) 2.12–3.02 (3.24) [2.52 ± 0.31], lacrymoid, thin-walled, hyaline, inamyloid. Basidia 16–26 × 4.2–9.4 µm, clavate, thin-walled, hyaline, 4 - (rarely 2 -) sterigmate; sterigmata 1.5–4.3 µm long. Basidioles 15–26 × 3.7–6.5 µm, mostly fusiform, occasionally clavate, thin-walled, hyaline. Cheilocystidia Rotalis - type, 12–24 × 4.5–8.3 µm; main body elongated-cylindrical to elongated-clavate; apex bearing 4–13 projections, projections 1.8–3.9 µm high. Pleurocystidia not observed. Lamellar trama irregular; hyphae 3.2–5.3 µm wide, thin-walled, hyaline, dextrinoid. Pileipellis hymeniform, composed of Siccus - type cells, 11–20 × 5.6–10 µm; main body spherical to broadly clavate; apex bearing 5–15 small spines, spines 3.4–5.7 µm long, pale brown in 5 % KOH. Pileus trama composed of irregular, interwoven, thin-walled, hyaline, dextrinoid hyphae 3.2–4.9 µm wide. Stipitipellis composed of parallel, smooth, thin-walled hyphae 3.9–6.7 µm wide. Clamp connections present in all tissues. Typus. CHINA • Inner Mongolia Autonomous Region, Tongliao City, Horqin Left Wing Rear Banner, Nugustai Town, on dead branches of coniferous trees, 14 July 2022, 43°23'48"N, 122°24'53"E, alt. 311 m, F. Guo & T. Bau, FJAU 69326 (gf 241), holotype!. Additional materials examined. CHINA • Inner Mongolia Autonomous Region, Tongliao City, Horqin Left Wing Rear Banner, Nugustai Town, on dead branches of coniferous trees, 14 July 2022, 43°23'48"N, 122°24'53"E, alt. 311 m, F. Guo, FJAU 69324 (B 04). • Heilongjiang Province, Greater Khingan Range Prefecture, Huma County, Guqigu National Wetland Park, on dead branches of broad-leaved trees, 13 August 2024, 52°19'49"N, 124°43'10"E, alt. 358 m, H. Cheng, FJAU 69325 (C 2481401). Notes. It is most closely related to Ma. falcatipes. Ma. falcatipes is distinguished by its generally eccentric, cylindrical, equal stipe, which is curved to geniculate, and by its Siccus - type pileipellis, the main bodies of which are cylindrical to clavate, often lobed and diverticulate (Desjardin and Petersen 1989). Ma. falcatipes typically grows on senescent leaves or stems of broad-leaved trees, herbaceous vines, or Poaceae plants, whereas this species grows on dead branches of coniferous or broad-leaved trees or on decaying wood. And Ma. elaeocephalus occurs solitarily on woody debris of undetermined dicotyledonous trees within coastal secondary forests or cacao / banana groves and is known from Africa (Grace et al. 2019). Ma. horqinensis occupies a habitat similar to that of Collybiopsis koreana (Antonín, Ryoo & H. D. Shin) J. Sung Kim & Y. W. Lim. Nevertheless, Co. koreana can be readily differentiated during field surveys by its finely tomentose pileus, particularly at the center, with distinct radial rugulose-sulcate features (Antonín et al. 2010 b; Kim et al. 2022). This new species is also closely related to Ma. conchiformis. However, Ma. conchiformis bears reniform to shell-shaped basidiomata, lacks a stipe, and produces clavate to naviculate or fusoid basidiospores measuring 8–11.8 × 3–5 µm (Oliveira et al. 2014).