Verrucaria macrostoma Dufour ex DC.

    Thallus crustose, usually rather thick, cracked-areolate (rarely subsquamulose at the edge), dried mud-like during dry periods, areola plane to slightly convex, 0.5-1.5 mm diam., smooth to faintly rough, dark brown to brownish when dry, olive-brown to olive-green when moist (and then, with green thallus, subsquamulose towards margin). Perithecia globular, numerous, 1 to 3 per areola, 0.3-0.5 mm diam., half immersed in thallus and half-projecting above the surfaces of the areoles, blackish. Ascospores simple, colourless, ellipsoid, 20-28 x 10-13 µm. Chemical spot tests negative. On hard calcareous rocks and, in our prospection area, mostly abundant on mortar of shaded and humid walls near the seashores. Can be confused with a close taxa, Verrucaria viridula that also grows on walls, usually dryer and with perithecia pear-shaped and smaller, deeply immersed in the thallus and less prominent. A different morph growing on damp walls with soredia and propagules at the areoles margin, can also be found: Verrucaria macrostoma f. furfuracea. See also Verrucaria nigrescens with colours drab and not turning green. N.B. See Verrucaria squamulosa Brand & van den Boom with a thallus formed by scales, sometimes isolated, with rounded edges and much greener. This species strongly resembles certain highly squamuloses forms of Verrucaria macrostoma but is genetically distinct from them, is sometimes difficult to separate macroscopically, and there appear to be all possible intermediate forms.