Thallus: erect-shrubby to ±pendulous, 2 to 25 cm long, stiff branching: isotomic-(basal part) to anisotomic dichotomous (extremities), generally divergent basal part: distinctly black pigmented that sometimes reaches the basal part of the main branches, with numerous and conspicuous annular cracks extending upwards branches: cylindrical or tapering; lateral branches: not narrowed at attachment points; foveoles and transversal furrows: absent segments: terete and cylindrical papillae: verrucous to sometimes cylindrical, irregularly distributed, mainly on main branches fibercles: absent fibrils: long (3-5 mm), sometimes numerous and giving fishbone-like aspect to the branches soralia: irregularly rounded to transversally elliptical, even, ±flat to slightly stipitate, raised and then concave, larger than half the diameter of the branch, rarely encircling the branch, remaining discrete, rarely becoming confluent, moderately dense to dense, especially on terminal branches isidiomorphs: present on young soralia, generally absent on mature soralia cortex: thick (9-14 %), dull, distinctly cracked at the base of main branches medulla: thin and compact, not pigmented axis: thick, not pigmented Apothecia: not seen Spot tests: K+ yellow turning red, C-, KC-, P+ orangish yellow Secondary metabolites: salazinic acid (major), and ±protocetraric acid (minor). Substrate and ecology: on bark World distribution: Europe, Macaronesia, eastern and western North America Sonoran distribution: Santa Cruz Island in southern California. Notes: The combination of the following characters makes this species distinct: basal part with black pigmentation extending to the lower parts of the main branches, with numerous and conspicuous annular cracks, thick cortex (9-14%), very thin medulla (7-12%), soralia larger than half the diameter of the branch and the presence of salazinic acid. Sometimes, however, the black pigmentation may almost be absent, or the annular cracks may not be so numerous or the soralia may be badly developed. In these cases, the chemistry is the only distinct character that separates U. silesiaca from U. subfloridana.