squamules: orbicular flat or concave on top, often angulate or lobulate, up to 4 mm diam. and 0.7 mm high upper surface: olive-brown to dark brown, rugose and fissured, not sorediate upper cortex: not developed but with an epinecral layer (10-30 µm thick) medulla: composed of loosely interwoven hyphae; algal layer: 140-200 µm thick lower cortex: paraplectenchymatous, 10-50 µm thick, composed of 4-7 layers of isodiametric cells lower surface: paler than the upper surface, attached by a tuft of hyphae Apothecia: one or two per squamule, centrally immersed, with slightly raised rim when mature; disc: dark red to almost black, expanded, up to 1 mm in diam.; epihymenium: K- or rarely K+ red-violet, >100-spored ascospores: globose, 5-8 µm diam. Pycnidia: not observed Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: on poor or sandy soils and on detritus in rock fissures in deserts and other open, arid habitats World distribution: North Africa, SW North America Sonoran distribution: so far only known from Maricopa County, Arizona. Notes: This species was not separated from P. polyspora (= P. patellata) by Wetmore (1970), but was subsequently separated from that species by Egea (1989) on the basis of differences in thallus structure. The squamules of P. patellata are somewhat smaller and usually half as thin as those of P. psammophila. In addition, P. psammophila has a thicker epinecral layer and a more lax medulla than P. patellata. Perhaps the best separating character is the fact that the epithecium is almost always K- in P. psammophila and K+ red in P. patellata.