Thallus: crustose to weakly placodioid, rimose-areolate to areolate-verrucose, usually forming flattened or slightly convex rosettes, up to 6 cm in diam., often coalescing, 0.5-2 mm thick centrally, tightly adnate except sometimes at the margin, even to uneven; prothallus: absent areoles: +distinctly separated, 1-3 mm in diam., often sinuous, flat to convex, often becoming swollen and plicate-foveolate lobes: usually distinct but mostly only weakly radiating, coarse, +isodiametric to moderately elongated, 1.5-2(-4) mm long, 1-2 mm wide, 0.5-1 mm thick, moderately convex and thickened throughout or becoming +flattened and broadened toward tips, simple and subentire to coarsely crenate and occasionally slightly branched upper surface: when fresh usually light to moderate greenish yellow, the outer 0.5 mm of lobe tips usually concolorous or pale greenish yellow (in herbarium becoming moderately to strongly yellow or grayish yellow), rather smooth, usually densely covered with yellow powder, esorediate upper cortex: with moderately conglutinate hyphae, without dead algal cells, rather even above, 25-40(-50) µm thick medulla: soft and crumbly, appearing cottony, moderately loose, with +randomly oriented hyphae; algal layer: +distinct, divided into glomerules lower cortex: absent Apothecia: often absent, scattered to sometimes crowded in center of thallus, borne submarginally, sometimes several per areole, soon sessile and constricted at base, 2-3(-4.5) mm in diam. disc: medium to deep yellow or yellow-green, pale red under pruina, flat to somewhat concave, densely pruinose margin: concolorous with thallus or somewhat paler, usually 0.3-0.7 mm wide, raised and inflexed at least when young, becoming flexuous to lobed, often crenate towards the inside, without a parathecial ring amphithecium: present, with algae (divided into irregular groups) filling most of amphithecium and extending as a layer (c. 50 µm thick) below the hypothecium, medulla filled with xanthone granules (soluble in K)and zeorin crystals (c. 30 µm long, 1-3 µm wide), corticate; cortex: similar to that of the thallus parathecium: pale yellowish brown, composed of dense, conglutinated hyphae very narrowly parallel hyphae, sometimes with small yellowish oil droplets epihymenium: with c.10 µm thick superficial layer of coarse, greenish gray to yellowish brown, refractive granules (partly soluble in K), partly penetrating deeper into hymenium hymenium: hyaline, 35-80 µm tall; paraphyses: coherent; tips: little thickened 1.5-2.5 µm; subhymenium: pale yellowish brown, c. 30 µm thick, often with oil droplets; hypothecium: mostly with periclinally to randomly oriented hyphae asci: clavate, 8-spored ascospores: hyaline, simple, oblong to oblong-ellipsoid, (8-)10-12 x 3-4(-5) µm Pycnidia: often common and somewhat conspicuous; conidiophores: type III of Vobis (1980) conidia: 8-12 µm long Spot tests: cortex K+ yellow, C+ orange, KC+ yellow-orange, P+ yellow; medulla K+ yellow turning brownish red, C+ orange, KC+ orange-red; discs C+ and KC+ orange-red Secondary metabolites: two main chemotypes: A) (holotype and most others): thiophanic acid and an unknown xanthone major, arthothelin absent or in traces only, UV+ blue unknown LPC-1 and traces of several other unknowns present; and B) (Guadalupe Island specimens): arthothelin major, thiophanic acid and other major xanthone absent or in traces only, and a different set of minor unknowns present. Substrate and ecology: on siliceous rocks (andesite, chert, lava), usually on exposed horizontal to sloping surfaces, often near tops of rocks, often very abundant World and Sonoran distribution: Baja California and Baja California Sur at 3-90 m. Notes: Lecanora pachysoma is distinguished from L. pinguis (found just north of our region) by having: 1) thinner, more flattened thallus (not removable intact), without a distinct hypothallus; 2) usually densely powdery surface; 3) yellower when fresh; 4) more strongly constricted apothecia with thick, prominent, often crenate margins; 5) medulla and lower hypothecium composed of +randomly oriented hyphae, such that the consistency is looser and cottony rather than chalky, 6) P+ yellow reaction of the cortex, absence of UV+ white substances LPN-1and 2, presence of UV+ blue unknown LPC-1, and other differences in chemical content; and 6) more southerly distribution (mainly in Mexico). Specimens of L. pachysoma that are poorly developed, damaged, or discolored, with rather thin and finely rimose-areolate thalli, can be very difficult to distinguish from L. xanthosora; see notes under that species. These two species differ from other Sonoran members of subg. Placodium in having a scarcely conglutinate upper cortex, elongated spores, and various other features.