Thallus: placodioid, irregularly rosette-forming (sometimes coalescing), rimose-areolate to verrucose-areolate centrally, closely and tightly attached throughout, 1-3 cm across, 0.5-1 mm thick centrally, thinner at margin areoles: closely contiguous to partly discrete, plane to slightly convex, 0.5-1 mm across, +round to +angular lobes: closely contiguous to +discrete, radiating, (0.5-)12(-4) mm long, (0.3-)0.5-1(-2) mm wide, plane to slightly or moderately convex, occasionally +lifted-folded (as in Lecanora muralis), entire to +coarsely crenate-incised towards tips, the ultimate segments 0.3-0.5 mm wide, appressed near the tips or somewhat set off from the substrate surface: appearing yellowish white to white or (especially when infected with black torulose hyphae) distinctly bluish white, light olive-gray to yellowish gray under pruina with light grayish yellowish brown showing under +rough pruina especially near the lobe tips, partly light to dark grayish yellowish brown or grayish brown at extreme lobe tips, dull, roughened and rimulose to finely verruculose, densely and coarsely pruinose throughout upper cortex: the upper 10 µm pale brown, appearing cellular, the lower part interspersed with fine granules (soluble in K), interspersed with coarse granules (insoluble in K), evenly 20-30 µm thick medulla: interspersed with grayish granules (insoluble in K), with hyphae c. 3 µm wide; algal layer: continuous to interrupted, 30-40 µm thick, with trebouxioid algal cells 10-15 µm in diam. lower cortex: absent Apothecia: common towards thallus center, dispersed, borne submarginally, one per areole, adnate to sessile, sometimes somewhat constricted but appressed, (0.6-)0.8-1.2 mm in diam. disc: dark gray, grayish brown, dark grayish yellowish brown, +plane, moderately to densely white pruinose when young, pruina fine to coarse, less pruinose when older or becoming bare, under pruina brownish black to olive-black or black margin: same color and texture as thallus, slightly to moderately raised when young, +soon level with disc, persistent, entire to flexuous-sinuous or coarsely crenate towards inside, (0.1-)0.2-0.3(-0.4) mm wide, without a parathecial ring amphithecium: present, with a continuous to interrupted algal layer (40-50 µm thick) in upper part of margin and extending under the hypothecium, with a chalky medulla below hypothecium filled with pale brown, coarse (c. 20 µm in diam.), globose crystals with a radiating pattern of elongated structures with acute tips (insoluble in K), corticate; cortex: indistinctly delimited, 30-50 µm thick, similar to that of thallus parathecium: hyaline, rather distinct, composed of conglutinated, parallel, thick-walled hyphae, c. 20 µm thick, continuing to the surface epihymenium: brown, interspersed with fine dark granules (soluble in K), N+ red, the surface covered with coarse granules (partly soluble in K, partly soluble in N), c. 30 µm thick hymenium: hyaline, I+ blue, c. 75 µm tall; paraphyses: loose in water, flexuous, branched towards tips, 1.5-2 µm thick; tips: clavate, 3-5 µm wide, with blackish pigment especially near the tip of the apical cell; subhymenium: poorly differentiated, c. 30 µm thick; hypothecium: hyaline, 100-400 µm deep, the hyphae randomly oriented asci: ±broadly clavate, 30-40 x 10-18 µm; apex Bacidia-type (according to E. Timdal) or Eiglera-type (according to C. Printzen), appearing almost completely I+ blue, with at most a low, indistinct axial body and ocular chamber; outer wall layer I+ blue; 8-spored; ascospores: hyaline, usually well-developed, simple to faintly 1-septate, narrowly ellipsoid to oblong-ellipsoid, (6-)8-13(-18) x 3-5 µm Pycnidia: not seen Spot tests: thallus K-, C-, KC-, P-, UV-; medulla KC + red-violet (fleeting) or (in material from Santa Catalina Island) KC- Secondary metabolites: lobaric acid, or (in material from Santa Catalina I.) unidentified phenolic (pale brown after charring 4-5A, 6&6B, 5C). Substrate and ecology: on +acidic rocks (including basalt and chert), at summit of hills or on north-facing slopes above washes, in coastal scrub communities, associated with a black-and-white mottled species of Verrucaria (same species as in L. lichenicola and L. brattiae) World and Sonoran distribution: coastal, southern California (San Nicholas Island, Santa Catalina Island and mainland of Los Angeles and Orange Counties) at 10-240 m. Notes: Lecanora albocaesiella is similar to Lecanora brattiae in many ways, especially in the lobate thallus containing lobaric acid and occurring on the seashore, but it is distinguished by a combination of features: 1) densely pruinose upper surface almost throughout; 2) somewhat thinner upper cortex, densely filled with coarse, hyaline crystals insoluble in K; 3) often densely and persistently pruinose discs which become black under the pruina; 4) usually wider or more raised thalline exciple; 5) lower hymenium and shorter, broader asci; 6) mostly clavate and narrow paraphyses tips; and 7) well-developed and somewhat larger ascospores. Lecanora albocaesiella is rather variable externally, especially in apparent color of the thallus and discs, and in development of the lobes and thalline exciple. Material of L. albocaesiella is superficially somewhat similar to Solenopsora candicans (Dickson) J. Steiner, and may be the basis for reports of that species from southern California; however, L. albocaesiella differs by having at most faintly 1-septate spores, Bacidia-type rather than Catillaria-type asci, a different chemistry, filiform conidia (Hasse's report of S. candicans stated that it has bacilliform conidia) and various other features of morphology and anatomy. The cortex type of L. albocaesiella (inspersed with gray crystals of calcium oxalate) suggests affinities with subg. Placodium sect. Pruiniferae s.l., the type species of which, Lecanora pruinosa Chaub., also has ascus tips that are almost entirely I+ blue, without the large apical axial mass typical of Lecanora (see Hawksworth and Dalby 1992); these and related species may eventually be recognized as a separate genus. Lecanora brattiae differs from the L. valesiaca and L. arenisaxciola (the two members of sect. Pruiniferae occurring in the Sonoran region) by having pruinose discs and occurring in coastal areas. Some specimens of L. albocaesiella have a distinct blue tinge to the thallus, but this color is due to black torulose hyphae of a parasite, whereas the basic color of the thallus (as reflected in the epithet albocaesiella) is basically white due to the dense pruina.