Thompson, J., 1997. American Arctic Lichens: The Microlichens.
Thallus glaucescent or ashy or whitish glaucescent or blackish, crustose, verrucose-uneven to subareolate or lacking. Apothecia to 0.8 mm, at first flat and marginate, soon convex and immarginate; exciple dark outside, violet within, radiate; hypothecium lower part dark, upper part red-violet to pale; epihymenium bluish or greenish; hymenium 50-55 µm, upper part bluish or greenish, lower hyaline or red-violet; paraphyses coherent, septate, 1.5 µm, tips capitate to 4-5 µm; asci cylindrico-clavate; spores uniseriate orbiseriate, fusiform, hyaline, 3-septate, 11-19 x 3-5 µm.
Reactions: thallus K—, C—, P—; epihymenium K—; hymenium K—, 1+ blue then wine-red.
This species grows on calcareous rocks, characteristically in shaded sites such as overhangs and river gorges. It is boreal to temperate but has been reported from the Arctic on Bear Island (Lynge 1926) and Novaya Zemlya (Lynge 1928). Fink (1935) reported its range as from Massachusetts to Iowa. In WIS are a number of specimens from shady cliffs in Illinois, Indiana, Ohio and Wisconsin. Wetmore (1967) added South Dakota to the North American range. The bimodality of the mapped range is puzzling, and more collecting needs to be done before a definitive range can be stated. My Alaskan report was erroneous (Thomson 1979). In a discussion of the lichens collected on the 1965 Foray in Wisconsin, Brodo commented (1967) that the type specimen of Bacidia granosa (Tuck.) Zahlbr. agreed in all respects with descriptions of B. trachona. In case these are indeed synonymous, the range would extend to Florida and California.