Guide Study Lich.: 181, 1898. Basionym: Collema nigrescens var. ryssoleum Tuck. - Lich. Calif.: 34, 1866.
Synonyms: Collema meridionale Hue; Collema nigrescens f. rupestre (Bagl.) Zahlbr.; Lathagrium nigrescens var. rupestre Bagl.
Distribution: N - Lig (Watson 2014, Giordani & al. 2016). C - Tosc (Pišút 1997, Tretiach & al. 2008), Laz (Tretiach 2004, Genovesi & al. 2011), Sar (Nöske 2000, Rizzi & al. 2011). S - Camp (Nimis & Tretiach 2004), Cal (Puntillo 1996), Si (Grillo & al. 1996, Ottonello & Romano 1997, Grillo 1998, Grillo & Caniglia 2004, Ottonello & al. 2011).
Description: Thallus foliose, homoiomerous, subgelatinous when wet, 1-6 cm in diam., adnate, broadly lobed. Lobes 0.5-1 cm wide, 50-85 µm thick when wet, rounded and often somewhat ascending at apex, densely pustulate and coarsely ridged, dark olive-green to brown or rarely black, often pale yellowish brown in old parts. Lower surface greenish brown, pitted with a few hapters. Upper and lower cortices absent. Apothecia lecanorine, crowded 0.5-1.5 mm across, with a flat to convex, brownish red to blackish disc, and an entire, persistent thalline margin. Proper exciple thin, euthyplectenchymatous; epithecium brownish; hymenium colourless, 85-110 µm high; hypothecium colourless. Asci 8-spored, clavate, the apex thick, with a I+ blue apical dome and a I+ blue annulus projecting downwardly. Ascospores 4-7-septate, hyaline, fusiform, straight or curved, 22-47 x 4-11 µm. Pycnidia immersed or semi-immersed, on both sides of the thallus, pale. Conidia bacilliform or with slightly swollen ends. Photobiont cyanobacterial (Nostoc), in long chains. Spot tests: all negative. Chemistry: without lichen substances.
Note: a mild-temperate, mainly western species in Europe, found on steeply inclined seepage tracks of moderately acid to basic siliceous rocks at low elevations; restricted to the Tyrrhenian region in Italy.