Immature cap parabaloid to ellipsoid, 3-10 (15) mm tall, 2-8 (10) mm broad, expanding to broadly-conic, 5-15 (20) mm broad; margin at first incurved, soon level, eventually revolute and torn; surface radially-striate, dark-grey, sometimes ochre-tan at the disc, covered in youth by a veil of white, deciduous hairs; context thin, watery grey, deliquescent; odor not distinctive, taste untried.
Gills free, crowded, narrow, pallid in youth, soon grey to black from maturing spores; edges even; lamellulae in one to three series.
Stipe 10-50 mm tall, 0.5-2.5 mm thick, fragile, round, hollow, slightly enlarged towards the base; surface of apex nearly glabrous, translucent-white, inconspicuously streaked from innate fibrils, the lower portion covered with scattered coarse hairs, these more concentrated towards the base; partial veil absent.
Spores 10.5-14.0 x 6.5-7.5 µm, cylindric-elliptical in face-view, similar in profile but inequilateral, sometimes bean-shaped, smooth, hilar appendage inconspicuous, with a central germ pore; spore blackish in deposit.
Solitary, scattered, or in groups on horse manure; fruiting throughout the year whenever moisture is available; common.
Unknown, too small to have culinary value.
This small inky cap fruits abundantly on horse manure during periods of moist weather. The delicate fruiting bodies emerge in the early morning, deliquescing within hours, often replaced by a fresh flush the next day. Coprinopsis radiata is recognized when young by bullet-shaped fruiting bodies sheathed in cottony fibrils. The expanded caps are broadly-conic, dark-grey, plicate-striate, and decorated with wisps of universal veil tissue. Although ephemeral, and frequently overlooked, it can be easily studied by collecting dung containing primordia, incubating it in a moist chamber.
Related species include Coprinopsis macrocephala, C. cinerea, and C. lagopus. The first two also occur on horse dung and have a similar veil, but are larger and more robust, the expanded caps of Coprinopsis macrocephala up to 4.0 cm broad, and more densely hairy, while those of Coprinopsis cinerea, up to 5.0 cm broad, with a proportionally thicker stipe. Coprinopsis lagopus can be distinguished by a lignicolous habit.
Besides the above, several distantly related small coprinoid fungi can also be found on dung. A microscope is required for their identification, but they can usually be differentiated from Coprinopsis radiata with a hand lens by examining the nature of the veil, if present. Coprinopsis ephemeroides, for example, is a diminutive species with a pale yellow cap disc covered with fine granules, remnants of a veil, and has an annulus; Coprinopsis cordisporus is virtually identical but lacks an annulus; Coprinellus pellucidus lacks a cap veil but has projecting pileocystidia; Parasola miser is beautiful tiny species with a copper-colored pileus in youth; it lacks both a veil and pileocystidia; finally Coprinopsis stercorea has a cap veil of furfuracous granules, and a fleeting unpleasant odor.