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 or darker and blackish, 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)-septate, 35-55 x 1.2-1.5 µm according to the literature; 42-50 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.

Cf.  Bacidia laurocerasi whose thallus is more bluish and the apothecia more black at the end.