Collema flaccidum (Ach.) Ach.
Collema rupestre (Sw.) Rabenh.
Thallus foliose, more or less rosette-shaped, composed of large lobes, 1-1.5 cm wide, often ascending, membrane-like, elasticated, thin and unchanged when humid, smooth, irregularly or little rumpled or wavy, covered with laminal or marginal isidia, more or less globose at first becoming squamulose or even for the larger ones scaly, 0.2-0.5 mm, dark greenish, olive-brown when moist, almost dark or soot-like when dry. Apothecia absent or very scarce, 0.5-2.5 mm diam., disc flat, brown-red, thalline margin persistent. Ascospores fusiform, 3-5-septate, 20-40 x 6-8 µm. Photobiont: Nostoc. Chemical spot tests negative. Rather common on humid and sheltered rocks near river banks, and also on vertical walls of humid rocks subjected to water seepage. See Collema subflaccidum that is rather similar but growing on bark of trees and on rocks. See also Collema furfuraceum of which lobes are more folded, rumpled and isidia coralloid, and Collema auriforme of which lobes and isidia swell when subjected to humidity, and Collema crispum with numerous secondary lobes.