Thallus: crustose, usually endosubstratal in non-sorediate parts and hence appearing totally leprose, usually thin, weakly delimited or indeterminate, at first forming small rounded patches up to a cm or more wide, later usually becoming irregular and often confluent with adjacent thalli and then sometimes covering extensive areas up to several dm wide, areolate; prothallus: often distinct, white or more rarely partly tinged blue, very rarely distinctly blue throughout giving the thallus a +blue over-all color esorediate areolae: sometimes distinct in richly fertile specimens, slightly convex, up to 0.2 mm wide, becoming +contiguous, ecorticate surface: mostly green or pale yellow, rarely deep yellow or green with a blue or gray tinge, sorediate soredia: farinose, diffuse, 20-50 µm in diam., often in aggregations, in variably formed soralia that usually protrude through the uppermost cell layer of the substrate or rarely burst from corticate areolae and that soon become confluent forming a leprose crust or more rarely are +persistently discrete and +orbicular up to 0.5 mm wide medulla: rarely evident, white Apothecia: lecanorine, usually sparse or absent, sessile, up to 1.5 mm wide disc: dull yellow, flesh-colored, pale pink or brown, flat to slightly convex, rarely distinctly convex, occasionally pruinose, up to 0.8 mm in diam. margin: not or only weakly prominent, often persistent, concolorous with thallus or lighter, soon becoming sorediate and/or excluded, without a parathecial ring amphithecium: present, with +randomly scattered algal cells, composed of irregularly entangled hyphae, laterally up to 85 µm wide, ecorticate parathecium: hyaline, 20-35 µm wide laterally epihymenium: pale yellow, with numerous small crystals soluble in K, 8-15 µm thick hymenium: hyaline, 35-55(-60) µm tall; paraphyses: 1.5-2.5 µm thick, simple to sparsely branched and anastomosing; with only slightly thickened apices; subhymenium: hyaline, 35-50 µm thick; hypothecium: hyaline to pale straw colored, 90-100 µm thick asci: cylindrical-clavate, 11-15 x c. 35 µm (often immature), 8spored ascospores: hyaline, simple, narrowly to broadly ellipsoid, (7.5-)9-13(-16) x (3-)4-6(-7) µm Pycnidia: not seen Spot tests: K+ pale yellow, C+ deep yellow to orange, KC- or intensified, P-; UV+ dull violet (short wave), dull brownish (long wave) Secondary metabolites: usnic acid, zeorin, thiophanic acid (major), unidentified xanthone(s), expallens unknown (Tønsberg 1992). Substrate and ecology: on bark and wood of shrubs and over plant remnants World distribution: Africa, Asia, Europe, New Zealand and North America Sonoran distribution: southern California and Baja California Sur at sea-level to 400 m. Notes: A morphologically similar, but chemically different form (from California, Santa Barbara Co., Santa Cruz Island, on bark of Pinus muricata, leg. Printzen) contains usnic acid, zeorin, and a major, unidentified substance at RF-classes A5, B (4-)5, C (5-)6 (bright yellow to yellow-green in LW UV-light before charring) may be related to L. expallens; however, it has only been found sterile and its affinity is at present uncertain. When sterile, L. expallens is easily confused with Pyrrhospora quernea, but that species is distinct, e.g., in having a thallus with a fawn tint and by not producing usnic acid and zeorin. This species was initially recognized by T. Tønsberg, who provided the initial description.