Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2004. Lichen Flora of the Greater Sonoran Desert Region. Vol 2.

Thallus: crustose, usually well developed, +regularly areolate to rimose-areolate, 0.2-2 mm thick; prothallus: black, obvious at the margin of the thalli or indistinct or lacking areoles: contiguous, flat to slightly convex, irregular to moderately regular, 0.2-2 mm in diam. surface: whitish gray to pale bluish gray, esorediate cortex: 10-17 µm thick medulla: white, I+ intensely violet, sometimes medulla-I reaction not uniformly throughout the thallus (but usually more distinct under the hypothecium; rarely medulla I-) Apothecia: black, sitting in between and usually not overtopping the areoles, 0.4-2(-3.5) mm in diam., singular or in sometimes large and dense groups (then outline of apothecia angular) disc: black, flat to slightly convex, dull, epruinose to (usually weakly) pruinose margin: very thin or disappearing exciple: with an epihymeniumlike brownish rim and an unpigmented (50-80 µm wide, but often strongly reduced) interior part epihymenium: olive-brown, dirty green, bright green (cinereorufa-green), or dark brown, 10-15 µm thick hymenium: hyaline, 4375 µm tall, I+ blue; paraphyses: not or rarely branched and usually not anastomosing subhymenium: distinguishable only after staining with lactic blue, 25-80 µm thick hypothecium: unpigmented to very pale green or pale yellowish brown asci: clavate, 38-55 x 9-15 µm, 8-spored ascospores: hyaline, simple, broadly ellipsoid (rarely sub-globose) to oblong-ellipsoid, (5-)6.2-13.5(-17.5) x (2.5)3.6-6.7(-8) µm, length-width-index 1.2-2.4, mean spore volume 70-235 µm3 Pycnidia: immersed, graphidoid conidia: bacilliform, (6-)9-13(-18) x 1-1.5 µm [studied in 28 specimens] Spot tests: cortex and medulla K-, C-, KC-, P- Secondary metabolites: with 3 sympatric distributed chemotypes of obviously minor taxonomic rank: chemotype A: with confluentic acid syndrome; chemotype Substrate and ecology: in open habitats on acid and slightly calcareous rocks, including sandstones World distribution: bipolar and extending into the tropics in alpine areas Sonoran distribution: widespread and locally common from mid-elevations to the alpine and upper montane zones of Arizona, California, Baja California, Baja California Sur, Chihuahua, and Sinaloa. Notes: Lecidea tessellata is a very widespread and rather polymorphic species. It is rather easily recognized by its regularly areolated, usually moderately to rather thick thallus with usually has a I+ dark violet medulla, its half-sunken apothecia with an mostly somewhat reduced exciple, its unpigmented hypothecium, and its small, thick-walled and blunt ascospores. The epitheton "tessellatus" (Latin) means "built of little square stones" (like a mosaic) and refers to the uniformly cracked areolate thallus. In alpine areas over calcareous rocks, one finds Lecidea tessellata var. caesia (Anzi) Arnold, characterized by a pale bluish gray thallus with a floury surface (see Hertel 1967). This variety is common and widespread in alpine areas with a more humid climate but not yet reported from the Sonoran region.