Rhizocarpon richardii (Lamy ex Nyl.) Zahlbr.

    Thallus crustose, potentially forming large mosaics, tightly encrusted on the rock surface, usually more or less circular, to 10 cm diam., areolate, cracked, network-forming, pale grey, grey, grey-brownish with grey-purple or grey-violet tinges, prothallus black, often large and well developed, markedly visible at the rim and between the areoles. Apothecia very numerous, 0.5-1 mm diam., inserted in the thallus, flat to slightly convex, surface uneven,, exciple thin, slightly paler, indistinct, black. Ascopores variable in shape and aspect (including from ascii and apothecia belonging to the same thallus), sometimes 1-septate  but often apparently submuriform with an incomplete secondary network suggesting muriform septa, but seldom truly muriform, colourless though some well mature specimen may become pale grey-brown, 22-27 (32) x 11-14 µm according to literature, 22-27 x 10-15 µm according to our measures. Photobiont: Chlorococcoid. C+ red (medulla), KC+ red (medulla), K+/- yellow, Pd+/- orange (Reactions vary depending on the gyrophoric, stictic and norstictic acid contents). Medulla: I+ indigo. Rather common on siliceous rocks, notably smooth ones, or on large pebbles on the foreshore where its aspect of irregular tiles, alternating dark and pale zones, often with purple tinge is characteristic of this species.

_ Not to be confused with Rhizocarpon reductum Th. Fr. that is C-, paler, smaller, less draughtboard-shaped, more common, with muriform ascospores and not especially maritime.

_ Not to be confused with taxa belonging to Buellia aethalea s.l. sharing the same type of well developed black prothallus but K+ yellow > red and with ascospores strictly 1-septate. On maritime rocks, see also Buellia stellulata  that is C-, K+ yellow and Pd+ yellow and much smaller (with 1-septate ascospores as in all Buellia species). 

_ It has been described Rhizocarpon richardii (Lamy ex Nyl.) Zahlbr. subsp. richardii which would have spores with a gray-brown submural appearance when mature and which above all would not be maritime. Currently this subsp. is not taken into account by the current authors.