Parmelia saxatilis (L.) Ach. forme saxicole.

    Foliaceous up to 10-15 cm in diameter, formed of lobes arranged in a rosette and overlapping (especially in the center), slightly appressed, short and truncated, 0.5-1 cm wide, upper surface as if embossed by a clear whitish network of pseudocyphellae, bluish, greenish-grey, bluish-grey, brownish at the end of the lobes and sometimes blackish in the center, covered from the center with isidia, first hyaline then whitish and finally darkening from the top to become completely grayish, these isidia being able to cover the thallus in its almost entirety, especially on very exposed rocks; lower surface dark brown, blackish with simple, blackish rhizines. Apothecia rare, 0.5-1 cm in diameter, disc dark orange-brown, margin isidiate. Broadly elliptical, simple, colorless spores, 16-18 x 9-11 µm. Cortex: K+ yellow; medulla: C-, K+ orange, KC+ orange, P+ orange. Not a particularly maritime species but common on acidic rocks. Taxons growing on rocks near the sea are often sterile and heavily covered with isidia, and the thallus is sometimes blackish in the center due to necrosis.

N.B. Beware of the saxicolous form of Parmelia sulcata forme saxicole, which is very similar in appearance but much rarer, and has soralia rather than isidia.