Description: Thallus foliose, heteromerous, dorsiventral, rather loosely attached, forming 4-20 cm wide, mostly regular rosettes. Lobes sublinear, contiguous, elongate, separate, flat, 1.5-2.5(-3.5) mm wide, with truncate apices. Upper surface grey (to brown in exposed situations), smooth to foveolate, somehow shiny, rarely slightly white-pruinose in marginal parts, with numerous, laminal, elongate-linear, white pseudocyphellae. Isidia cylindrical, simple to coralloid, often darkened at tips, mainly laminal and most abundant in central parts of thallus. Lower surface black, with simple to furcate, black rhizines present also along the margins. Upper cortex paraplectenchymatous, of 2-6 layers of densely packed cells, with a usually non-pored epicortex, the cell walls with isolichenan; medulla white; algal layer continuous; lower cortex paraplectenchymatous, thinner than upper cortex. Apothecia rare, lecanorine, 2-8 mm across, with a brown disc and a usually isidiate thalline margin. Epithecium brownish; hymenium and hypothecium colourless. Asci 8-spored, clavate, Lecanora-type. Ascospores 1-celled, ellipsoid, hyaline, (11-)13-15(-18) x 6-8(-11) µm. Pycnidia black. Conidia cylindrical to bacilliform, 5-8 x 1 µm. Spot tests: upper cortex K+ yellow, C-, KC-, P+ yellow; medulla K+ yellow turning orange-red, C-, KC-, P+ orange-red, UV-. Chemistry: upper cortex with atranorin and chloroatranorin; medulla with salazinic acid (major) and consalazinic acid (minor), and sometimes traces of lobaric and protocetraric acids; fatty acids (lichesterinic and protolichesterinic acids) absent.
Note: a mainly saxicolouos species whose real distribution and ecology in Italy are very poorly known. Many Italian records could refer either to P. ernstiae (especially epiphytic samples), or to P. serrana (especially saxicolous samples). The statement by Castellani & al. (2021) that P. saxatilis is the most widespread species of the complex in Italy is dubious, being merely based on an unreliable morphological analysis of samples, without confirmation by molecular data.
Growth form: Foliose, broad lobed
Substrata: bark and rocks
Photobiont: green algae other than Trentepohlia
Reproductive strategy: mainly asexual, by isidia, or isidia-like structures (e.g. schizidia)