On the type of Coprinus marcidus
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Original description
Coprinus marcidus Van De Bogart in Mycotaxon 8: 262. 1979 (The genus Coprinus in Western North America, part 2: sect. Lanatuli). Holotype: North America: state Montana, Flathead, 8 sept. 1972, F. Van De Bogart 2185 (WTU).

Pileus at first glandiform, then conic, then plano-convex and finally revolute. Prior to expansion 4.0-5.0 mm in length and after expansion 10.0 mm in breadth. Pale grey or white at first under a dark grey to greyish black universal veil layer, then becoming pale grey with white striae where the surface overlies each lamellar trama, and dark grey at the apex. Surface becoming deeply plicate-striate. Universal veil at first obscuring the pileal surface with a continuous fibrillose dark grey to almost black layer that eventually splits into shredlike remnants and becomes widely scattered with a few larger fragments at the pileal apex. Flesh thin at the apex. Stipe slender or filiform, 3.5 cm x 0.6-1.1 mm, tapering slightly from base to apex, hollow but with the lumen containing a column of water. Pale grey to glassy straw color, opaque at base to semitranslucent at apex. Surface covered with coarse and strigose grey fibrils at base and part way up the stipe, apical half glabrous. Flesh extremely thin, watery, and fragile. Lamellae narrowly lanceolate or ellipsoidal, with some lamellulae, 2.0-4.3 x 1.0 mm, free from stipe apex but with all lamellae attached to a rudimentary collarium. Growded at first then subdistant as plicate striation develops with pileal expansion, pale then soot black. Autodigestion incomplete, only the margins affected by lysis. Odor and taste none.
Spores ellipsoidal, 8.7-13.8 x 6.2-8.3 x 6.2-7.5 µm, mostly round in cross-section but with some larger spores slightly flattened, apiculus large and prominent, germ pore apical, 1.2-1.5 µm in diameter. Color en masse soot black microscopically deep purplish black 3% KOH., walls smooth. Basidia trimorphic, short clavate and 20.0-22.0 x 10.0 µm, long clavate and 30.0-32.0 x 10.0 µm, ululiform and 35.0-36.5 x 8.6-10.0 µm, all four-spored. Cheilocystida probably present but not observed due to partially lysed condition of lamellar margins. Pleurocystidia few in number, located near the lamellar margins only, ellipsoidal, 60.0-80.0 x 23.0-26.0 µm, apices rounded, hyaline, thin walled, smooth. No other cystidia present. Pileal surface of radially elongated cells, two to three times as long as broad, cell walls pale brown in 3% KOH. Universal veil present on pileus and at stipe base as a dark fibrillose layer that splits into small irregular parches but not into small upturned scales. Fibrils and patches composed of chaines of elongate cells 50.0-100.0 x 10.0-27.5 µm, hyaline to pale brownish, thin walled, swollen, consticted at the septa, usually with clamp-connections, unbranched. Some of the chains of cells present at stipe bases are little or not swollen and resemble normal cylindrical hyphae. Clamp-connections present on universal veil on stipe, in pileal trama, and in the lamellar trama.
Habitat. Lignicolous, on a piece of wood buried in a thick moss carpet on the floor of a conifer forest. Solitary.

This species possesses several distinctive features, of which the dark color of the veil and the pronounced absence of autodigestion except the lamellar margins are the most outstanding ones. More subtle but still distinctive characters are the pleurocystidia that are rather small and few in number, and the slightly colored cells of the pileal surface which are elongated radially but do bear some resemblance to the appearance of a cellular type of surface.

Observations
Spores [20,1,1] 11.2-12.3 x 7.7-8.5 µm, ellipsoid or ovoid, with slightly conical base and truncate apex, and central to slightly eccentric, c. 1.6 µm wide germ pore, dark red-brown; Q = 1.35-1.55, av. Q = 1.45; av. L = 11.7, av. B = 8.1 µm. Basidia c. 26 x 10 µm, 4-spored, surrounded by 3-5 pseudoparaphyses. Cystidia collapsed. Pileipellis hyphoid. Veil made up of hyphoid, thin-walled elements, 40-180 x 7-32(-40) µm, in part somewhat inflated, constricted at septa; terminal cells clavate, ellipsoid or subfusiform. Clamp-connections present, small.

Notes: The collection exists of only one small basidiocarp with very dark veil.
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