Collema subflaccidum Degel.

    Foliose thallus arranged in a rosette of 4-10 cm in diameter, formed of broad lobes, generally few in number, 0.5-1.5 cm wide with rounded apices, blackish in dry weather, dark greenish, dark olive and slightly gelatinous in humid weather (but not swelling significantly in water); covered with globose isidia especially towards the centre, sometimes more or less coralloid towards the margin (these isidia have a sooty appearance and never become squamulous). Apothecia absent or very rare, red-brown disc and more or less isidiate margin. Spores long fusiform, 5-7 times septate, 42-55 x 5-7 µm. Photobiont: Nostoc. Thermophilic species growing on the rough bark of deciduous trees in humid and shady woods, very rarely lignicolous or saxicolous.

_ If the isidia become squamulous see: Collema flaccidum.

_ If the isidia are cylindrical or coralloid and the lobes strongly pleated see: Collema furfuraceum.