Collema nigrescens (Huds.) DC

   Folliaceous thallus more or less pleated up to 10 cm in diameter, lobes large, up to 1 cm wide, covered with more or less radial veins made up of smooth pustular bubbles (toad skin like!), dark olive-brown, blackish if dry, bottle green or olive if moist. Presence on the upper face, especially towards the centre, of small blackish globular isidia 0.2 mm in diameter. Apothecia usually few and localized but sometimes covering almost the entire thallus, 0.4-1 mm in diameter, disc flat or slightly convex, pale brown slightly pinkish to dark brown, sometimes pruinose  white, with a narrow, at times isidiate, thalline exciple. Spores long spindle-shaped, needle-shaped; 5-8-(10)-septate, 50-90 x 3-4.5 µm ; according to the literature; 45-65 x 3.5-4.5 (5-5.5) µm, 5-8 (9) septate according to our measurements. Photobiont: Nostoc. Chemical spot tests negative. Species with strong ocean tendencies coming at the base of trees or in crevices of rocks or walls (especially churchyards by the sea).

Cf. Collema subnigrescens very similar (and often confused!) has a larger and much more radially pleated thallus, lacks isidia and more numerous apothecia, and especially its spores are somewhat smaller and less wide and 4-5-(6)-septate.