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Remarks (internal):Leucocoprinus fragilissimus is widely distributed in the tropics and subtropics (Halling & Mueller 2005). It is characterized by subdeliquescent basidiomata and a membranous, yellowish-white pileus with small yellow squamules (Pegler 1983, Rother & Borges da Silveira 2009). Although the hymenial elements were not observed in our collections due to the subdeliquescent pileus, the basidiospore shape and size and pileipellis structure completely agree with those of this species.
According to Pegler (1983) and Smith & Weber (1982), L. fragilissimus is probably a common pantropical species but rarely collected owing to the extremely delicate basidiomata, which make field collection difficult. The presence of cheilocystidia in this species is rarely mentioned. According to Smith & Weber (1982), the cheilocystidia are 13-25(-36) x 9-15(-20) µm in Leucocoprinus tenellus Pegler [nom. illegit., non (Boud.) Locq.] from the Lesser Antilles and L. thoenii Heinem. from Zaire are also species with thin fragile basidiomata. Their habit and spore shape also resemble L. fragilissimus, but L. tenellus and L. thoenii differ in the absence of any yellow pigments (Pegler 1983). Leucocoprinus brunneoluteus Capelari & Gimenes from Brazil also has fragile basidiomata but differs from L. fragilissimus in its more yellowish coloration and the presence of brown umbo and squamules (Capelari & Gimenes 2004, Rother & Borges da Silveira 2009).
 
Description type:Non-original description 
Description:Leucocoprinus fragilissimus (Ravenel ex Berk. & M.A. Curtis) Pat., Essai Tax.
Hyménomyc.: 171 (1900). Figs 6-8
≡ Hiatula fragilissima Ravenel ex Berk. & M.A. Curtis, Ann. Mag. Nat. Hist., Ser. 2, 12: 422 (1853).
Pileus 20-44 mm diam., conic-campanulate, becoming plane with a small umbo, even depressed, sulcate-striate to the umbo, white to slightly yellowish (1.A1-1.A3) when young, becoming whitish and almost translucent except for a central disk in maturity, umbo pale yellowish-brown (1.A4) with small yellow squamules, dry. Lamellae white, thin, subdistant, free, with lamellulae; margin entire. Stipe 60-100 x 1-2 mm, equal, fragile, central, light yellow (1.A5), with scattered small yellowish squamules, fistulose. Annulus present, white, small, membranous, movable, evanescent, attached to the upper to middle part of the stipe. Context white, very thin, almost absent, except at the disk, membranous, subdeliquescent. Spore print off-white.
Basidiospores 9-12.7 x 6.5-10 µm (11.1 ± 0.8 x 8.1 ± 0.6 µm); Q = 1.1-1.6; Qm = 1.4, n = 54, broadly ellipsoid to subglobose, smooth, hyaline, thick-walled, with an obvious germ pore with a hyaline dome, dextrinoid, metachromatic, cyanophilic. Hymenial elements not observed. Pleurocystidia and Cheilocystidia unobserved. Pileipellis an epithelium with hyaline, globose, more or less isodiametric elements, 17.2-25.3 µm in diameter, thin-walled, mixed with hyaline, thin, more or less cylindrical hyphae. Stipitipellis a cutis of narrowly cylindrical, 5-12 µm broad hyphae. Clamp connections absent. Habitat (of Argentinean collections) — Solitary or gregarious in small groups. On decomposing dicotyledonous wood and in soil among litter in subtropical pastures and forests.
Distribution - Leucocoprinus fragilissimus has a pantropical distribution (Pegler 1983). It has been recorded from Africa, Asia, Europe, North America, and South America (Ecuador: Reid et al. 1981; Brazil: Albuquerque et al. 2006, Capelari & Maziero 1988, Gimenes 2007, Pegler 1997, Rother & Borges da Silveira 2009, Wartchow et al. 2008). This is the first record of this species in Argentina (Misiones and Corrientes Provinces).
 
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