Rhizocarpon infernulum forma sylvaticum Fryday
Rhizocarpon oceanicum ?
Thallus relatively thin, smooth to weakly rimous, sometimes slightly bordered or sometimes slightly cracked with age in some thicker areas, pale brownish, chocolate, etc., hypothallus usually present blackish. Apothecia numerous, scattered throughout the thallus, becoming sessile with age, 0.3-0.6 mm in diameter, disc flat to slightly convex, black, margin clear, thin and persistent, concolorous. Colorless spores, 1-septate, 17-20 x 9-11 µm according to the literature, (15) 17-20 x 9-11 µm according to our measurements. Thallus: C-, K-, KC-, P-. Uncommon species occuring on humid siliceous rocks and in the shade of forests under oceanic influence, seems to be characteristic of granite rocky chaos along the riverbank of humid and wooded valleys in Brittany. N.B. Very similar (and formerly confused) species of Rhizocarpon hochstetteri which has larger and longer spores (> 21 µm) and which is more mountainous.