Bot. Notiser: 303, 1939. Basionym: Collema isidioides Nyl. ex Arnold - Flora, 53: 232, 1870
Description: Thallus small-squamulose to subfruticose, black, subgelatinous when wet, forming up to 3 mm wide, irregular rosettes, the central parts of squamules with ascending to erect, isidia-like, 0.1-0.2 mm wide, 0.4-0.5 mm long, cylindrical lobes. Apothecia rare, developing from pycnidia, laminal, subimmersed, up to 0.5 mm across, with an expanding, brownish disc and a persistent, smooth thalline margin. Proper exciple very thin, of parallel hyphae; epithecium brownish; hymenium and hypothecium colourless, the hymenium I+ reddish brown, K/I+ blue; paraphyses mostly simple, coherent, not apically thickened. Asci 8-spored, prototunicate, cylindrical, without a distinct apical apparatus. Ascospores 1-celled, hyaline, broadly ellipsoid, 10-13 x 7-8 μm. Pycnidia rare, immersed. Conidia simple, bacilliform, hyaline, 1-2 x c. 1 μm. Photobiont cyanobacterial (Nostoc, in chains), the individual cells 4-6 μm wide. Spot tests: K-, C-, KC-, P-, UV-. Chemistry: without lichen substances.
Note: on periodically wetted surfaces of various types of rocks; widespread in Northern Europe, further south most frequent in the mountains; known from scattered localities throughout the Alps, outside the Italian territory. To be looked for in the Italian Alps.