Cap at first globose to hemispheric, expanding to nearly plane, 4.0-7.0 (9.0) cm broad at maturity; margin incurved, then decurved, plicate-striate, soon lacerate, in age either recurved or deflexed; cap disc covered by a felty, pallid to buff-brown, stellate universal veil patch, often encrusted with sand or debris; cuticle pallid, thin, torn by expansion into radially arranged whisp-like fibrils; context membranous, blackish, plicate; odor pleasant, of mushrooms; taste untried.
Gills free, close to crowded, deliquescing, blackish in age.
Stipe 6.0-10 cm long, 0.5-0.7 (1.0) cm thick, equal except for a marginate basal bulb, the latter underlain with tan-colored fibrous mycelium; stipe hollow at maturity, thin-walled, readily splitting, with a central cottony thread; surface whitish, appressed fibrillose, or with patchy squamules, becoming greyish near the apex from maturing spores; partial veil when present fugacious; annulus seldom seen at maturity.
Spores16.0-19.0 x 9.5-11.0 µm, elliptical in face-view, slightly inequilateral in side-view, smooth, germ pore eccentric, up to 2 microns in diameter, hilar appendage usually conspicuous; spores blackish in deposit.
Solitary or in small groups in sparse grass and sandy soils along the coast; also in arid, sagebrush country east of the Sierra Nevada; uncommon.
Befitting its species name, Coprinus calyptratus has a distinctive, often star-shaped cap patch, a remnant of the universal veil. This feature plus a stipe with a marginate bulb help to separate it from a closely related cousin, Coprinus comatus. Small specimens of the latter are superficially similar, but can be distinguished by a veil, that breaks up into erect cap scales, the presence of an annulus, and microscopically by spores that are smaller, usually with a central germ pore. Another close relative of Coprinus calyptratus, is C. xerophilus. This smallish species, like Coprinus calyptratus, is found in dry habitats, particularly in the Great Basin, and has a well-developed cap veil. The veil, however, is whitish, not buff to tan, often fragments, and tends to occupy a greater portion of the cap surface. Both species have relatively large spores with eccentric germ pores.