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, membranous and adnate, broadly lobed, 1-6 cm in diam. 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 older parts. Lower surface greenish brown, with a few, scattered, white hapters. Upper and lower cortices absent. Apothecia common, lecanorine, crowded, 0.5-1.5 mm across, with a flat to convex, brownish red to black disc and an entire, persistent thalline margin. Thalline exciple usually pseudocorticate; proper exciple thin, euthyplectenchymatous; epithecium brownish; hymenium colourless, 85-110 µm high, I+ blue; paraphyses coherent, simple or sparingly branched, 2-3 µm thick at mid-level, the apical cells swollen; hypothecium colourless or pale yellow. 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, ellipsoid to fusiform, straight or slightly 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, the cells 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.