Closed pileus globose, subglobose, ellipsoid or cylindrical ellipsoid, up to 12 mm high and 8 mm wide, completely covered with powdery, pale pinkish brown veil (Mu. 7.5 YR 5/4, K.& W. 6C4 at centre) which forms small conical flocks at centre of pileus; at margin, particularly in early stages, with somewhat more hairy-floccose veil; expanded pileus up to 25 mm wide, conical or convex, later applanate. Lamellae, L = 18-24, l = 0-3, free, up to 1.5 mm wide, white at first, then greyish to black. Stipe up to 60 x 0.5-1.5 mm, white, somewhat hyaline, at base clavate, up to 2.5 mm wide, often brownish, with white velar flocks, often building a small, volva-like, erect collar. Smell absent.
Spores [240,12,12] 7.3-11.6 x 6.5-10.1 µm; Q = 0.95-1.25, av. Q = 1.05-1.20; av. L = 7.9-10.2, av. B = 7.1-9.4 µm, rectangular lemon-shaped, lentiform, dark red-brown, with central germ pore. Basidia 12-32 x 7-10 µm, 4-spored (sometimes 2-spored and than spore-size about equal to that of the 4-spored collections), surrounded by (3-)4-6 pseudoparaphyses. Pleurocystidia 40-80 x 16-28 µm, utriform, subglobose to ellipsoid or subcylindric. Cheilocystidia 20-50 x 17-32 µm, utriform, subglobose to ellipsoid or subcylindric, mixed with lageniform ones (20-50 x 8-12 x 3-5 µm). Veil made up of (sub)globose to ellipsoid elements, smooth to granular, up to 50 µm wide. Clamp-connections absent.
The most distinctive character of Coprinus cordisporus is the presence of lageniform cheilocystidia.
Coprinus cordisporus is very similar to C. patouillardii. Many authors consider them to be synonyms (Kühner & Romagnesi, 1953; Enderle, Krieglsteiner & Bender, 1986). Orton & Watling (1979) differentiate Coprinus cordisporus from C. patouillardii on habitat (pure, relatively fresh dung), smaller sized fruitbodies (pileus 5-10 mm), smaller spores, and presence of pleurocystidia. C. patouillardii has a larger pileus (10-20 mm), and grows on kitchen refuse, silage, straw and soil mixed with old dung. Arnolds (1982) accepted the taxa on account of presence or absence of pleurocystidia, but expressed his doubt as to the validity of these concepts. Citerin (1992) also accepted two taxa, based on presence or absence of pleurocystidia and a difference in width of the spores. To the present authors the differences indicated above are not strong enough to warrant a distinction on specific level between Coprinus cordisporus and C. patouillardii. Size of the fruitbodies and spores is rather variable in this complex.
Coprinus patouillardii ssp. isabellinus Locq. differs only by the smaller basidiocarps and smaller spores. In all other microscopical characters it is similar to C. cordisporus, especially by the presence of lageniform cheilocystidia. Therefore, we consider it a synonym of Coprinus cordisporus rather than a subspecies of C. patouillardii.
Coprinus cordisporus is also very similar to C. cardiasporus, and somewhat less to C. ephemeroides. It differs from both by the lageniform cheilocystidia. Coprinus ephemeroides has a small annulus, its size is somewhat smaller, and the pileus is more yellowish. C. cardiasporus has heart-shaped spores.
The lageniform cheilocystidia of Coprinus cordisporus are sometimes branched at apex, which was clearly visible in coll. Jalink 1299 and Bender, 7 Oct. 1988. Also on the stipe similar caulocystidia have been found, mixed with globose velar elements.