Verrucaria hydrela  Ach.

    Thallus crustose, thin, smooth, gelatinous and greenish when immersed, slightly cracked or areolate, greyish, brown-green when dry (N.B. this thallus, when scratched, reveals an underlying black basis). Perithecia numerous, scattered all over the thallus, conical, little prominent with a rounded apex, projecting 0.3-0.5 mm, leaving a pit when broken. Ascospores very broadly ellipsoid, simple,  17-25 x 7-10 µm. Chemical spot tests negative. Aquatic species thriving in freshwater on calcareous or weakly acid rocks or boulders in rivers and streams, immersed in winter and only uncovered in summer.

N.B. Taxon variously interpreted according to the authors.

_ See Verrucaria praetermissa which grows on acidic rocks less submerged at the edge of the river, with a more whitish and strongly cracked thallus and which has narrower spores.