Bacidina inundata (Fr.) Vezda

    Thallus crustose up to 5-6 cm in diameter, thick, more or less smooth to  irregular and rough surface, grainy, sometimes a little cracked, green if damp, gray-green, almond-green, yellowish-green or brownish if dry, often with a whitish hypothallus. Apothecia not always present, more or less round, 0.3-1 mm in diameter, at the beginning a little included in the thallus then slightly sessile, disc more or less flat, at the beginning light brown then greyish, dark brown and at the end blackish-brown, paler margin visible only at the beginning and disappearing at the end. Needles-haped spores, 3-(7) septate  25-45 x 2-3 µm. Pycnidia more or less immersed in the thallus. Photobiont: Chlorophyceae. Chemical spot tests negative. On periodically inundated acid rocks near streams or in unpolluted streams and rivers.

NB Easy to confuse with Bacidia carneoglauca = Aquacidia antricola very similar and from the same habitat but the latter species has a more luminous green thallus, a larger white hypothallus, sessile apothecia with a persistent paler or whitish margin and very numerous pycnidia, sessile, white and cottony.