Verrucaria prominula Nyl.
Thallus crustose, very thin, evanescent, dull white, dull grey, grey-green (paler then the rock below), most often limited to an arachnean network characterized by its colour: grey-purplish to pale brown-mauve, sometimes absent on very friable rocks, prothallus poorly developed or absent. Perithecia usually large, prominent, and well developed (to 0.8 mm diam.) broadly conical, scattered, apex truncated, flattened or rounded, somewhat broadly opened below, resembling to an old volcano, rarely out of shape except in highly exposed areas. Simple, oblong ascospores, sometimes constricted in the center, typically with rounded ends, 12-15 x 6-8 µm according to the literature, 13-17 x 5-7 µm according to our measurements. Rare or understated unless specifically searched, on vertical, shaded faces or in crevices on schist rocks located in the spray zone and the seawater projection zone on seashores.
N.B. Species relatively easy to identify by its maritime habitat on shales or soft, slightly calcareous rocks, its almost absent thallus, its large perithecia and its spores with rounded tips.