Cap 4.0-10.0 mm broad at maturity, at first ellipsoid to ovoid, then obtuse-conic, expanding to plane, the margin becoming recurved and torn; surface white to pale-grey, striate-plicate to near the disc, mealy-granulose, hairy toward the margin, in age the hairs inconspicuous; context thin, membranous, weakly deliquescent; odor unpleasant; taste untried.
Gills adnexed, subdistant, narrow, pallid in youth, eventually blackish; lamellulae in up to two series.
Stipe 1.0-3.5 cm long, 0.5-1.0 mm thick, more or less equal, hollow, fragile; surface watery-white to pale-grey, when young covered with whitish hairs, soon sparse, except at the base; partial veil absent.
Spores 6.5-8.0 x 3.0-3.5 µm, smooth, moderately thick-walled, cylindrical-elliptical in face-view, similar in profile but slightly inequilateral, hilar appendage inconspicuous, germ pore central; medium-brown mounted in KOH; spores blackish in deposit.
Scattered to clustered on herbivore dung, especially that of horses and cows; fruiting throughout the mushroom season; common.
Coprinopsis stercorea is one of several small Coprinus species (sensu lato) that occur on horse and cow dung. It is most easily recognized in youth when the unexpanded cap is covered with a universal veil of sugar-like or mealy granules and marginal hairs. An unpleasant, often fleeting odor, is also characteristic. With maturity, most of the cap hairs disappear which can cause confusion with Coprinopsis ephemeroides and Coprinopsis cordisporus, two species that also have granulose caps. Coprinopsis ephemeroides differs in possessing a pale-yellow cap disc and thin annulus. Coprinopsis cordisporus is best identified by a study of the spores and veil. The veil of Coprinopsis stercorea consists of globose, thin-walled, finely-warted cells interspersed with narrow, branched hyphae, the spores cylindrical-elliptical in face-view. In contrast the veil cells of Coprinopsis cordisporus are globose, thin-walled, smooth, the spores pentagonal to heart-shaped in face-view. Other common dung-inhabiting Coprinus species include Coprinopsis radiata, similar in size to Coprinopsis stercorea but with a veil of cottony, wispy fibrils, (not granulose); Coprinopsis cinerea, a larger species, the expanded cap up to 4.0 cm broad, also with a veil of cottony fibrils, and Coprinellus pellucidus, diminutive like Coprinopsis stercorea, with a veil-less cap, the surface instead ornamented with cystidia, these sometimes visible with a strong hand lens.