Thallus: shrubby to subpendant, 2-10 cm long, compact to loose branching: anisotomic-dichotomous, divergent basal part: usually conspicuously blackened on 2-4 mm, rarely only on a narrow and irregular zone branches: ±irregular to fusiform (secondary branches); lateral branches not narrowed at attachment point, but sometimes ±fusiform; usually with numerous annular cracks (4-8/0.5 cm) and conspicuous with white rings of medullary tissue (primary branches, close to basal part); often with distinct transversal furrows segments: terete (primary branches), often and characteristically slightly ridged giving to the branch an angular aspect in surface view (secondary branches) or flattened papillae: cylindrical, few to numerous, irregular to dense, on main and secondary branches tubercles (young stage of fibrils?): usually absent to numerous (especially on terminal branches bearing apothecia), ±large and verrucous, conspicuous, irregular, rarely eroded fibercles: present mainly on secondary and terminal branches bearing apothecia fibrils: slender, 1-6 mm long, usually numerous, regularly distributed in fishbone-like pattern soralia and isidiomorphs: absent pseudocyphellae: present on fibrils growing on branches or at the edge of apothecia cortex: moderately thick to thick (8-11 %), mat to ±shiny medulla: dense to compact, unpigmented axis: usually larger than medulla Apothecia: 2-13 mm in diam., terminal or lateral ascospores: broadly ellipsoid, 9-11.5 x 5-7 µm Spot tests: K+ dull yellow turning reddish orange, C-, KC-, P+ deep yellow, or K-, C-, KC-, P- Secondary metabolites: norstictic acid (major), ±fatty acids (minor), or rarely none detected. Substrate and ecology: on bark (Quercus spp. and Pinus spp.), rarely on wood, in oak, pine-oak forests or thorn forests (southern area), between 1500 and 2600 m, especially on windy crests of mountains World distribution: western coast of North America Sonoran distribution: mountains of Arizona, Baja California Sur, Sinaloa, Chihuahua, and Sonora. Notes: Morphologically, U. myrmaiacaina is close to U. intermedia (irregular branches with ridged segments) but it differs in its spore size and the chemistry, as well as in the width of the cortex, the numerous fibrils arranged in fishbone-like pattern and the distinct annulation pattern in the basal part. Anatomically, U. myrmaiacaina is close to U. florida (spore size, width of the cortex) but differs in having slightly irregular secondary branches with segments that are often ridged, cylindrical papillae as well as a different distribution range. Specimens collected in the southern and south western part of the Sonoran Desert area, with large spores and norstictic acid, can be for the moment only tentatively assigned either to U. myrmaiacaina or to U. florida. More detailed field and taxonomic studies are needed.