Verrucaria nigrescens Pers. var. nigrescens f. nigrescens

    Thallus crustose, rather thick but superficial, cracked, areolate, areoles smooth, flat to slightly convex, angular with black edges, 0.2-0.8 mm, smooth to rough, brown-black, blackish, blackish slighly tinged with green when moist, colours more or less drab and not glossy, incompletely green when moist, margin well developed, distinct, prothallus absent. One or several perithecia per areole,  1/2 to 1/3 immersed in thallus and therefore little protruding, small, 0.2-0.5 mm diam., black. Ascospores simple, broadly ellipsoid, 19-26 x 11-13 µm. Chemical spot tests negative. Not especially maritime, common on calcareous rocks and occasionally on mortar of walls or concrete. See Verrucaria macrostoma of which the areoles and perithecia are bigger and markedly turning green when moist and, Verrucaria viridula more brown-greenish and less blackish, appearing less areolate owing to a less visible prothallus, and with less protruding perithecia. See also Verrucaria fusconigrescens  that grows on siliceous rocks on the foreshore and has a more shiny thallus and Hydropunctaria maura that also thrives on the foreshore but is much blacker with broad and flat perithecia. N.B. An isidiate form (or blastidiate according to some authors) has been described: see Verrucaria nigrescens var. nigrescens f. tectorum that is easily confused with Verrucaria macrostoma f. furfuracea which is sorediate and markedly greener when moist. N.B. Verrucaria nigrescens s.l. is highly variable and probably encompasses several taxa.