Key to Danish species of Didymium Schrad.

1 Lime crystals on the peridium agglutinated into a smooth, egg-shell like outer layer. 2
1' Lime on the peridium as loose crystals or if agglutinated the the shell is wrikled and fragile. 3
2   (1) Spores finely warted, 11-14 µm in diam. D. difforme (Pers.) Gray
2' Spores with rough, scattered warts, occasionally with short lines, 9-10 µm in diam. D. trachysporum G. Lister
3   (1) Plasmodiocarps and/or sessile sporocarps. 4
3' Stipitate sporocarps. 9
4   (3) Plasmodiocarps effused, often as perforated plates. 5
4' Sporocarps and short, sausage-shaped plasmodiocarps. 6
5   (4) Capillitium with large, rounded vesicles filled with a yellow material. Spores 8-11 µm in diam., finely warted. D. serpula Fr.
5' Capillitium witout yellowish vesicles. Spores 10-12 µm in diam., with fine spines which may form an irregular net. D. dubium Rostaf.
6   (4) Columella absent or at most as a ± distinct and ± limy thickening of the hypothallus. On leaves of Petasites hybridus. 7
6' Columella present. Flattened to hemispherical. With or without white lime. On other substrates. 8
7   (6) Lime crystals on peridium ochraceous, giving the sporocarp a brownish appearance. Spores 9-10 µm Ø. D. vernum Kuhnt, K. Baumann & Nowotny
7' Lime on peridium white. Sporocarps appearing grey. Spores 10-15 µm Ø. D. tussilaginis (Berk. & Broome) Massee
8   (6) Columella heispherical, brown. The outer peridial layer colourless with brown spots. Sessile forms of D. melanospermum (Pers.) T. Macbr.
8' Columella irregularly flattened to hemispherical, white from lime. Inner peridial layer colourless. Sessile forms of D. squamulosum (Alb. & Schwein.) Fr.
9   (3) Stalk short, less than 1 1/4 times the sporocarp diameter. 10
9' Stalk long, at least 1 1/2 times the sporocarp diameter. 13
10   (9) Sporocarps disc-shaped. Columella absent. Spores 6-8 µm in diam., pale in transmitted light. D. clavus (Alb. & Schwein.) Rab.
10' Sporocarps ± spherical, often with an umbilicate base. Columella present. Spores 8-12 µm in diam. 11
11   (10) Stalk white from lime. Columella hemispherical, white to beige. Inner peridial layer uniformly colourless. D. squamulosum (Alb. & Schwein.) Fr.
11' Stalk black, devoid of lime. Columella hemispherical to flattened, brown to black. Peridial inner layer colourless with brown spots. 12
12   (11) Spores 10-12 µm in diam., dark brown in transmitted light, spinulose. D. melanospermum (Pers.) T. Macbr.
12' Spores 8-11 µm in diam., pale to medium brown in transmitted light, with fine warts and scattered groups of larger warts. D. minus (Lister) Morgan
13   (9) Stalk black. Peridium colourless with brown spots. Columella ± spherical, brown. D. nigripes (Link) Fr.
13' Stalk in upper part yellowish to orange, translucent, darker in basal part. Peridium uniformly colourless to yellowish. Columella a flattened thichening of the sporocarp base, whitish. D. bahiense Gottsb.