Cap 4.0-6.0 cm broad, ovate, then paraboloid, becoming hemispheric to broadly obtuse-conic, rarely expanding to convex; margin overlapping the gills, incurved, decurved in age; surface viscid when moist, drying glossy, wrinkled, cream, to pale-tan, darker at the disc, gradually fading at maturity; context white, unchanging, soft, up to 7.0 mm thick at the disc, gradually tapered towards the margin; odor not distinctive, taste "fungal."
Gills ascending-adnate to adnexed, close, broad, up to 10 mm in width, dingy greyish-tan when young, the edges lighter than the faces, minutely fringed with a hand lens, becoming mottled blackish, blackish overall in age; lamellulae in two series.
Stipe up to 15 cm long, 0.5-1.0 cm thick, slender relative to the cap, equal except enlarged at the base, the core narrow, stuffed; upper surface conspicuously pruinose-striate, cream-colored, lower portion appressed fibrillose, colored like the cap or dingy yellowish-tan, dry to lubricous when moist; stipe base when fresh, covered with a white pubescence, drying down and becoming tomentose; partial veil cottony-fibrillose, white, forming a fragile superior annulus, at maturity flattened to the stipe, blackened by spores.
Spores 15.5-20.5 x 9.0-12.5 x 10.0-11.0 µm, broadly elliptical in face-view, slightly narrower in profile, thick-walled, smooth, germ pore eccentric, the apex truncate, hilar appendage inconspicuous; spores grey-black mounted in water; spore print black.
Solitary or in small groups on horse and cow dung; fruiting throughout the mushroom season; occasional.
Questionable. (Arora: "edible according to most sources"; Jordan: "inedible"; McIlvaine: "excellent in substance and flavor"; Miller: "poisonous-hallucinogenic"; Phillips: "not edible"; Smith & Webber: "edible and good"; Stamets: "conflicting reports on the edibility of this species")
A large, statuesque mushroom, Panaeolus semiovatus is recognized by a viscid, cream-colored, wrinkled cap and annulate stipe. Its preference for fruiting on dung, particularly horse dung is shared with a smaller, more common cousin, Panaeolus papilionaceus. The latter is distinguished by a rust-brown to grey-brown, nonviscid cap, lack of annulus and a partial veil that leaves fragments on the cap margin rather than a ring. Unrelated but somewhat similar in appearance to Panaeolus semiovatus is Volvariella speciosa. It also has a light-colored, viscid cap when moist, but doesn't occur on dung. Other differences include the presence of a volva, lack of an annulus, and salmon-pink spores.