Bacidia arceutina (Ach.) Arnold
Thallus very thin, sometimes hardly visible, smooth or minutely granular or rimose (under magnifying lens), pale green-grey or grey. Apothecia usually numerous, scattered, immersed initially then ± sessiles, 0.2-0.6 mm diam., discs at first flat and marginate but often becoming convex, to a little globulose in old age, pale brown to brown-black, almost dark when old, concolorous margin, thick and clearly visible at the beginning then disappearing with time and finally immarginate. Hymenium section colourless (under the microscope). Asci with 8 spores, long utriform, 50-70 x 8-10 µm, according to our measurements. Spores needle-shaped (acicular), 3 to 8 (10) times septate, 35-55 x 1.2-1.5 µm according to the literature; 25-53 x 1.5-2.5 µm according to our measurements. Photobiont : Chlorphycea. Common but almost unseen due to its small size; on trunks, branches and leafstalks of trees and shrubs, and also (but rarely) on plant debris.