Opegrapha vulgata (Ach.) Ach.

    Thallus crustose, thin, smooth, whitish, pale brown, brown-orange, pale brown-reddish but most often found with these colours in combination, sometimes greenish-gray, see: Opegrapha vulgata forma. Apothecia lirelliform, simple, seldom branched or stellate, more or less immersed in the thallus, usually superficial and apparently segmented, 0.5-2.5 x 0.15-0.25 mm, black, disc at the bottom of a very thin or closed  minute slit. Ascospores hyaline, with a perispore, fusiform, straight or slightly curved with sometimes one tip slightly swollen, 4-7-septate (most often 6), 19-30 x 3-4 µm. Pycnidia numerous distributed over the whole thallus and clearly visible in the form of black dots in relief; pycnidiospores of several kinds but most often long and curved "sickle", 10-15 x 1 µm. Chemical spot tests negative. Rather frequent on smooth bark of tree trunks in humid and shaded woodlands. See Opegrapha niveoatra of which lirellae are shorter, around 1 mm and conidia short and curved and Opegrapha lithyrga very similar and sometimes synonymized, with a thin, greenish thallus, which occurs on damp vertical rocks and in shade.