Porpidia irrigua Orange
Porpidia contraponenda (Arnold) Knoph & Hertel p.p.
Thallus crustose, relatively thick, more or less smooth but most often areolate and cracked, areoles polygonal, a little convex, finely papillate to sometimes warty, whitish, dirty white, grayish-white, sometimes grayish-bluish between the areoles, prothallus often present blackish to bluish gray but sometimes clearly visible if in mosaic. (N. B. The thallus is sometimes lumpy and deformed by whitish galls of a parasite: Cecidonia xenophana). Apothecia 1 or 2-3 per areole, at first submerged then rapidly sessile, 0.3-2 mm in diameter, disc blackish, flat to slightly convex, sometimes pruinose, margin smooth, concolor, relatively thick and smooth, persistent. Simple, elliptical, colorless spores, 16-21 x 8-10 µm. Frequent pycnidia. Photobiont: chlorococcoid. No characteristic colored chemical reactions but presence of 2'-o-methylmicrophyllinate. Species found on low-acid and humid siliceous rocks (schists, dolerite, etc.) rather in hot areas and at low altitude, relatively frequent by the sea in Finistère where it seems to have been collected under the name Porpidia contraponenda.
N.B. Recently separated species from Porpidia contraponenda (with which it can cohabit), but this last species has more mountainous affinities, a thicker thallus with apothecia submerged in the thallus not becoming sessile and finally a slightly different chemical composition. Also difficult to separate from Porpidia cinereoatra whose prothallus is visible between the areoles and whose apothecia are partially submerged in the thallus, also difficult to separate from Porpidia platycarpoides whose thallus is dark gray.