Wahlenbergiella striatula (Wahlenb.) Gueidan & Thüs

    Thallus crustose, smooth, shiny, very thin at first and almost translucent, slightly gelatinous, with numerous rough ripples forming angular or polygonous ridges, dark green to blackish, 0.1-0.6 x 0.05-0.15 mm, and with black dots, scattered  and ± cracked; mature and well developed thalli have colours ranging from green, greenish, dark green, bottle green, to black towards the center resulting from piles of ripples (the black ripples are seen only in well-lit specimen) and according to exposition greenish and blackish zones may be found on the same thallus (see Wahlenbergiella striatula forme à stries polygonales and Wahlenbergiella striatula forme verte and Wahlenbergiella striatula forme noire), prothallus absent or effuse. Perithecia, conical, prominent, often irregular, 0.2 x 0.3 mm diam. polygonal towards the base with an irregular open ostiole on top, this ostiole can be very open to the point of making the perithecia appear like flattened volcanoes: Wahlenbergiella striatula forme à périthèces ouverts à l'apex. Pycnidia occasionally present, visible as minute black dots. Ascospores simple, ellipsoid, 8-11 x 5-6 µm according to literature, 9-11 x 5-6 µm according to our own measures, very small comparatively to medium sizes within this familiy. Uncommon, on rocks beaten by the waves in the barnacles ( (Balanus sp.) and the crustose red algae Hildenbrandia rubra zones, beneath the Verrucaria (Hydropunctaria) maura and Hydropunctaria (Verrucaria) amphibia zone and most often above the Verrucaria (Wahlenbergiella) mucosa  zone, this latter species is smooth, green but has a whitish prothallus, also in the Verrucaria halizoa zone, sometimes hardly distinguishable but with non angular perithecia and thalli without ripples and ridges.

N.B. Rather difficult to distinguish on the field from Verrucaria (Hydropunctaria) amphibia that grow a little bit higher on the foreshore but this last species is always very black with lace-like margin, see in particular Hydropunctaria (Verrucaria) amphibia forma.

N.B. Not to be confused with lush forms of Collemopsidium elegans of which the thallus is smaller, brown, and ascospores 1-septate.