Closed pileus subglobose to ellipsoid, up to 30 mm high and 18 mm wide, completely covered with grey or creamy-grey, powdery, mealy-floccose, often somewhat pointed veil, at centre of pileus sometimes rather dark sepia-brown at the tips of the scales; expanded pileus up to 40 mm wide, conical or convex, finally applanate with slightly deflexed margin. Lamellae, L = 24-38, l = 1-3(-5), free, white at first then grey to black. Stipe up to 110 x 4 mm, attenuate upwards, whitish; at base up to 6 mm wide, often brownish, with white velar flocks. Smell absent. Spore print fuscous black.
Spores [20,1,1] 13.4-16.7 x 12.7-15.4 x c. 8.5-10 µm, Q = 1.05-1.20, av. Q = 1.10, av. L = 15.1, av. B = 13.7 µm, lentiform, limoniform in frontal view, ellipsoid in side view, dark red-brown, with slightly to strongly eccentric germ pore. Basidia 18-38 x 9-13 µm, 2-spored, surrounded by 4-6 pseudoparaphyses. Pleurocystidia 80-160 x 30-60 µm, vesiculose, utriform, ellipsoid or subcylindric. Cheilocystidia 30-90 x 20-50 µm, similar to pleurocystidia. Veil made up of up to 90 µm wide, (sub)globose elements. Clamp-connections present.
Collection examined. UNITED KINGDOM: Inverness-shire, Nettle Bridge, 28 Aug. 1969, Orton 3555 (holotype, E).
The macroscopical description is based on the original diagnosis by Orton (l.c.); the microscopical data are based on our study of the holotype.
According to the description by Orton (1972), Coprinus pachyspermus can be readily recognised by the 2-spored basidia and the large lentiform spores. Although the pileus is described as grey or cream grey (Orton, l.c.; Orton & Watling, 1979), Orton & Watling in their key (1979: 16) characterized the species as 'white or grey to clay-buff scaly.' If the species exists also truly white, the only difference between C. pachyspermus and C. niveus is in the 2-spored basidia.
Orton (l.c.) did not mention the clamp-connections. We found some in the type material, but like in the other species in the 'C. niveus'-complex, they are difficult to find and not abundant.