Description.Thallus corticolous; uppersurface pale greenish yellow, dull, emaculate, not cracked; sparsely to abundantly isidiate, parts of the surface often gnarled and ± granular, where the isidia eventually emerge; isidia marginal, sublaminal to laminal, granular to cylindrical, rarely sparsely branched, 0.05–0.1(–0.2) mm in diam., concolorous with the thallus or brown to blackened at their tip, eciliate; lobes small to moderate-sized, 2–5 mm wide, rotund, undulating, axils incised, margins eciliate to very rarely sparsely ciliate; cilia, if present, short and slender, 0.3–0.5(–1) mm long, black, mostly simple, very rarely branched; lowersurface often blackened throughout or becoming dark brown towards the margin, typically with an erhizinate 0.5–1(–1.5) mm marginal zone; rhizines long, slender, black, mostly simple, rarely sparsely branched; medulla white. Apothecia and pycnidia not observed among the Galapagos specimens.
Ecology and distribution. Africa and North, South and Central America (Hale 1965); in South America previously known from Brazil (Hale 1965; Ribeiro 1998; Canêz 2005) and Venezuela (Vareschi 1962). In Ecuador so far only reported from the Galapagos, first by Weber (1986), subsequently by Elix & McCarthy (1998) and recently online (Bungartz et al. 2016); an uncommon corticolous species known only from a few collections on several islands, in exposed habitats, from the dry zone into the transition zone; typically growing on trunks and branches of Bursera trees, one specimen from Clerodendrum scrub (both Bursera and Clerodendrum are native to the Galapagos).
Notes. See the description of P. cactacearum for a discussion of how these two isidiate species with usnic acid can be distinguished.