Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2004. Lichen Flora of the Greater Sonoran Desert Region. Vol 2.
Thallus: crustose, well developed, coarsely warted, with elevated, flattened, +crenulate warts or +areolate surface: gray isidia: frequent especially when ascomata absent, +globose, forming irregular and unbordered but well-delimited, blue- or brown-black, +convex, soralia-like clusters; individual isidia: short-stalked and covered by a dark cortex cortex: uniform, rather thin, without strands of hyphae penetrating into the medulla, borderline of medulla even, parallel with surface fertile verrucae: 1.5-2(-2.5) mm in diam., with rough upper surface Apothecia: rare, immersed in older warts mazaedium: 0.5-1 mm in diam., plane, commonly greenish yellow pruinose exciple: not sclerotized, poorly developed laterally, forming a very thick, blackish brown cushion at the base; upper part: very thin and colorless; hypothecium: blackish brown, 140-220 µm high asci: persisting for a relatively long time, cylindrical, with uniseriately arranged spores, 50-67 x 5-8 µm ascospores: dark brown, 1-septate, somewhat constricted at the septum, broadly oblong, when ripe 22-28 x 12-15 µm; surface: somewhat irregular, sometimes appearing in the light microscope with a fine streaked pattern Spot tests: thallus K-, C-, KC-, P-; medulla I+ dark blue Secondary metabolites: thallus occasionally with usnic acid, atranorin, or norstictic acid; mazaedium and edge of exciple with rhizocarpic acid and epanorin. Substrate and ecology: on nutrient-enriched wood, especially on tops of fence posts, gate rails, or shingles (usually sterile) adjacent to fields and pastures enriched by nitrogen and other nutrients, wharves, stumps, also on dry sprouting twigs and stems of Pinus or Larix in less nutrient-rich sites at higher elevations (sometimes fertile) World distribution: South Africa, Europe and western North America Sonoran distribution: southern California at 1370 m. Notes: Thelomma ocellatum may be a secondary, asexual species derived from T. occidentale. See McCune and Rosentreter (1995) for discussion of the distribution and ecology of this species in western North America. Fertile specimens are very rare.