Candelariella medians (Nyl.) H. Magn.

     Thallus placodioid most often arranged in a circular and regular rosette of 2-4 cm in diameter, lobes of the margin tight against each other and continuous, flat or slightly convex, a little pruinose, yellow if dry, gray-yellowish a little greenish if moist (there is a lime-green form steepholmensis O. Gilbert that comes in admixture with the type), the thallus center strongly tends to become granular-warty with granular isidia and take on a yellowish-gray to gray-blackish and even to disappear completely which gives a cockade aspect to the thallus. Apothecia rare or even absent, 0.5-1.5 mm in diameter, flat disc, yellow-brownish, more or less crenate or granular margin disappearing rapidly. Hyaline spores, simple (sometimes with one septa) quite variable in shape but most often elliptical, 12-18 x 5-7 µm. C-, K-, KC-, P-. Species with thermophilic and nitrophilic tendencies coming on humid limestone rocks and even more often on limestone walls or concrete. Cf. Caloplaca decipiens, the thallus of which is sorediated and more orange in color and especially K + purple. Quite similar Caloplaca verruculifera but with more elongated marginal lobes is maritime and K + purple.