Description: Differs from related Psilocybe species mainly by the absence of annulus and hemispheric to hemispheric-convex pileus without an umbo or papilla.

Macroscopic feat.:

• Cap/Pileus: 26-38 mm diameter, at first conic or hemispheric-conic with incurved margin, then hemispheric to hemispheric-convex, not umbonate or papillate, dark yellow (4B4; Buff-Yellow) to yellowish brown (4B8; Aniline Yellow), sometimes darker in centre, often becoming paler towards the edge and almost whitish at margin, becoming beige when dry; surface smooth, slightly viscid when moist; margin even, entire, decorated with somewhat fugacious, white to bluing, membranous veil remnants.
• Gills/Lamellae: Adnate-sinuate, 4-6 mm high, close or subdistant, yellowish waxy-white when young, becoming pale grayish yellow with purple-brown (8E5) mottles when matured, then olive ochre (2C5) to dark brown, edges serrulate and remaining whitish, turning bluish during drying or when handled.
• Stem/Stipe: 50-68 mm x 3.5-4.5 mm, equal, slightly flexuous, longitudinally striate, hollow and fragile, white with sheen, becoming beige when dry; surface somewhat bluish when touched or bruised, or on drying, covered with white floccose scales which become bluish when touched; base of stipe with white mycelium; annulus absent.
• Context: Of pileus yellowish (2A3,Celonial Buff). Of stipe concolorous with that of pileus, whitish towards the surface. Some mycelium in centre of colony bluish in culture.
• Odor: Mild.

Microscopic feat.:

• Basidio/Spores: (241/5/2) (12-) 13-16 (-19) x (7-) 8-10.5 x 7.5-10 (-11) µm, often ellipsoid, elongate-ellipsoid to subhexagonal, occasionally ovoid, broadly ellipsoid, subrhomboid or subfusiform in face view, Q = (1.4-) 1.5-1.8 (-2.4), Q = 1.63±0.13; ellipsoid, subellipsoid to nearly oblong in side view, Q = (1.4-) 1.5-1.8 (-2.0), Q = 1.65±0.10, yellowish brown with purple tinge in water, yellowish brown in KOH, dark purplish brown in deposit; wall smooth, slightly thick to thick (0.5-1.5 µm), complex, with distinct 1-2 µm wide apical germ pore.
• Basidia: (24-) 30-44 (-53) x 8-14 µm, hyaline and colourless, occasionally yellowish, clavate to broadly clavate, narrowed in lower half or with a short to very long base 8-26 (-30) µm x 2-6 µm wide, 4-spored, sometimes 2-spored; sterigmata 3-6 (-8) µm.
• Cheilocystidia: 22-42 (-46) x 5-10.5 µm, hyaline, narrowly clavate, clavate or lageniform-clavate, sometimes lageniform or fusoid-ventricose, occasionally similar to pleurocystidia, but narrower, with a (2-) 3-11 x 1.5-4 µm subcapitate tip or neck, rarely forked.
• Pleurocistidia: relatively rare and scattered, 24-42 (-48) x (8-) 9-15 µm, thin-walled, hyaline, ventricose to broadly clavate and fusoid-clavate, often narrower near the apex, obtuse to short rostrum (2-4 x 2-4 µm).
• Caulocystidia: 23-54 x 6-10 (-14) µm, clustered at the upper part of the stipe, thin-walled, hyaline, some of them similar to cheilocystidia, narrowly clavate to lageniform, some oblong-subclavate, with short rounded rostrum, subcapitate apex or neck, 2.5-8.5 (-9.5) x 2-4.5 (-6) µm, sometimes irregularly branched. Clamp connections common in all parts of the basidioma.
• Subhymenium: Subcellular, composed of irregular vesiculose to subglobose cells, 3-13.5 x 2-12 µm, hyaline, colourless to yellowish, thin-walled to slightly thick-walled (about 0.5 µm).
• Trama: Regular, with cylindrical hyphae 2-13 µm diam, hyaline, colourless to yellowish, thin-walled to slightly thick-walled (≤ 1,0 µm).
• Epicutis: An ixocutis, 20-200 µm thick, made up of creeping, hyaline and colourless, subregular to interwoven, 1-4 µm wide filamentous hyphae, occasionally with dark or blackish finely incrusting pigments; subpileipellis dark yellow in KOH, composed of subregular, colourless to yellowish, filamentous to inflated, 2-16 µm µm wide hyphae.

Distribution: Chuxiong, Yunnan Province, China.

Habitat: Growing solitary to scattered and gregarious on cow dung or grassland where cattle have grazed in summer and early autumn.

Studied material: CHINA, Yunnan Province: Chuxiong City, Zixi Mountain, on cow dung, 25°01'05.5"N, 101°23'19.9"E, elevation 2237 m, 20 August 2009, Tao Ma CX026 (IFRD414-010, holotype). CHINA, Yunnan Province: Chuxiong, Wuding County, near a reservoir at road side in Bailu Village, on soil of grassland where cattle have grazed, 25°39'25.6"N, 102°05'30.0"E, elevation 2523 m, 24 Sep 2012, Xiao-Fei Lin WD007 (IFRD414-011)

Discussion: Etymology refers to the location Chuxiong, where the collections were found.
Psilocybe chuxiongensis is characterized by its subtropical habitat where it grows on dung. The pileus is smooth, buff-yellow to yellowish brown, hemispheric to hemispheric-convex without an umbo or papilla, lacks an annulus, while the context is yellowish both in the pileus and stipe. Bluing mainly occurs on the surface of the lamellae and stipe when bruising and drying, and mycelium are blue in culture. Microscopically Psilocybe chuxiongensis has large ellipsoid, elongate-ellipsoid to subhexagonal basidiospores 13-16 x 8-10.5 x 7.5-10 µm with slightly thick to thick walls, and hyaline, broadly clavate or ventricose pleurocystidia with an obtuse to short rostrate apex.
Following the taxonomical concept of Psilocybe proposed by Singer (Singer, 1948, 1986, Singer & Smith 1958), Psilocybe chuxiongensis belongs to sect. Caerulescentes Singer because of its surface bluing fruit body and blue mycelium in culture. In this section, it is closely related to Psilocybe cubensis as it has similar basidiospores, basidiomata, pleurocystidia and cheilocystidia, as well as the same habitat, and they can be easily differentiated from other species by the color and form of basidiomata, large spores and the subtropical habitat on dung. The relationship with Psilocybe cubensis is strongly supported by phylogenetic analyses in all three datasets; they cluster together as a sister group and can be distinguished from other members with high statistical support values.
Guzmán (1983) introduced sect. Cubensae to accommodate Psilocybe cubensis and Psilocybe subcubensis, which is characterized by coprophilous or subcoprophilous habitat, bluing basidiomata, subhexagonal thick-walled (0.5-1 µm) basidiospores, and a well-developed annulus. These two species are very similar and differ in the size of the basidiospores. Morphologically Psilocybe chuxiongensis can be easily separated from Psilocybe cubensis and Psilocybe subcubensis by the absence of annulus and the hemispheric to hemispheric-convex pileus without an umbo or papilla. Furthermore, Psilocybe cubensis and Psilocybe subcubensis have a white context, ovoid, broadly ellipsoid to subhexagonal or hexagonal basidiospores in face view, smaller ventricose pleurocystidia and fusoid-ventricose to lageniform to tibiiform cheilocystidia (Singer & Smith 1958 Guzmán 1983, Noordeloos 1999, Bau & Sarentoya 2009).

source - Phytotaxa [156 (4): 211-220]

Pictures at www.mushroomobserver.org