Closed pileus up to 23 x 18 mm, dark reddish brown to yellow-brown or leather- to cinnamon-coloured at centre (Mu. 2.5 YR 2.5/4, 5 YR 3/2-3, 5/8, 7.5 YR 4-5/6, 10 YR 5-6/5-6), paler towards margin (7.5 YR 4-5/6, 10 YR 6/7, 7/6, 5-7/4, 5-6/2, 5/1), up to c. 40 mm in diam. when expanded. Lamellae narrowly adnate to free, white to blackish; L = c. 26-28, l = 1-3. Stipe 40-100 x 1-3 mm, white to greyish-white, pubescent.
Spores [160,7,7] 9.3-14.1 x 6.2-8.2 µm, av. L = 10.3-12.3, av. B = 6.3-7.6 µm, Q = 1.40-1.85, av. Q = 1.50-1.65, ellipsoid to ovoid; germ pore eccentric, c. 1.8 µm wide. Basidia 18-43 x 9-10 µm, 4-spored. Pseudoparaphyses (4-)5-6(-7) per basidium. Cheilocystidia from globose and up to c. 50 µm in diam. to lageniform and 40-70 (-100) x 13-17 µm with somewhat tapering neck and 4-8 µm wide apex. Pleurocystidia 50-75 x 20-40 µm, vesiculose to utriform, not always present. Pileocystidia 60-120 x 13-24 µm, lageniform with cylindrical to somewhat tapering neck and 5-10 µm wide, equal to slightly broader apex. Sclerocystidia in most collections present (not found in type material). Clamp-connections present.
Coprinus subimpatiens is close to C. callinus. The most important difference between these two species is the shape of the cheilocystidia: in C. callinus exclusively globose, in C. subimpatiens intermixed lageniform and globose.
In species with predominantly globose cheilocystidia, like C. callinus, a few lageniform ones are sometimes present. In some species with mixed globose and lageniform cheilocystidia, the latter are not always abundant. Therefore identification is not easy. Fortunately there are other characters that may help.
In the case of C. subimpatiens and C. callinus such characters are: (1) the spores of C. subimpatiens are somewhat broader (6.2-8.2 µm versus 5.7-7.4 µm); (2) the neck of the pileocystidia is more (sub)cylindrical in C. subimpatiens and tapering towards the apex in C. callinus; (3) in C. subimpatiens sclerocystidia usually are rare or even absent, in C. callinus almost always present, sometimes abundant.