Protoblastenia calva (Dicks.) Zahlbr.
Protoblastenia rupestris var. calva (Dicks.) Lettau
Thallus crustose well delimited but not lobed at the periphery, very thin and sometimes not very visible, not cracked, on the surface or endolithic, whitish, whitish dirty, pale grayish to grayish (by cyanobacteria). Apothecia strongly prominent above the thallus, very rarely encrusted in a small shallow cavity or in pits, 0.5-2 mm in diameter, without margin and strongly convex, surface smooth and shiny, yellow-orange, orange-brown, red-brown, sometimes with a whitish or blackish border at their base. Spores largely elliptical, colorless, simple, 9-16 x 6-10 µm according to the literature and 11-15 x 7-10 µm according to our own measurements. Photobiont: green algae. Thallus: C-, K-, KC-, P-; apothecia: C-, K + purple, P-. Rather rare species occurring on the vertical faces of hard limestone rocks. See Protoblastenia rupestris, a much more common species, with flatter and smaller apothecia and a thicker and more or less cracked thallus, which occurs on old damp walls in alkaline stones or acid stones that have become alkaline. See also certain species of Caloplaca with marginalized apothecia and different spores.