Thallus: rimose-areolate, usually forming a determinate effigurate thallus, up to 4-5 cm or more in diam., irregular in shape inner areoles: angular 0.3-1 mm across, thin, mostly 0.2-0.3 mm thick, swelling as apothecia form outer areoles: same diam., usually with two or more distinctly small rounded lobes, but often becoming thicker, the edge becoming detached and areole convex, especially when coming in contact with an uneven substrate or another lichen; rim: down-turned surface: chalky white with fine pruina, yellow apparent when wet, smooth to slightly rough or uneven cortex: paraplectenchymatous, 40-60 µm, obscure in water, pale yellow, upper layer finely granular and adherent algal layer: thin, up to 100 µm thick, the cells wide-spaced medulla: obscured by granules, prosoplectenchymatous, continuous with attaching hyphae attachment: broad, without forming a stipe, sometimes elevating areoles Apothecia: one or two in each areole, punctiform to open, 0.1-0.5 µm wude, becoming somewhat elevated with a thin thalline margin disc: yellow; smooth, pruinose parathecium: indistinct, not expanding epihymenium: yellow, c. 10 µm thick hymenium: orange in thick section, hyaline in K, 80-110 µm tall; paraphyses: c. 2 µm wide at midlevel, apices barely expanded subhymenium: 15-25 µm thick; hypothecium: narrow asci: clavate, few, 50-70 x 10-15 µm, 100+-sprored ascospores: hyaline, simple, subglobose to broadly ellipsoid, 2-4 x 2-3 µm Pycnidia: not observed Spot tests: UV+ orange, K+ yellow slowly to red, usually forming crystals Secondary metabolites: norstictic acid (major), rhizocarpic acid (minor), connorstictic acid (trace), gyrophoric acid (trace), epanorin (trace). (HPLC, J.A. Elix, pers comm.) Substrate and ecology: limestone and calcareous sandstone World distribution: western North America (Texas); Sonoran distribution: Arizona. Note: Acarospora calcarea is a thin effigurate species appearing white in the field but becoming quite yellow when wetted like the non-effigurate A. epilutescens but differing from that species by the presence of norstictic acid. Acarospora.rouxii has a similiar chemistry, but differs in preferring acidic and volcanic substrates and in becoming quite thick and rugulose with a usually epruinose upper surface. Internally it is similiar to A. strigata. The specific name refers to its "chalky" look. Acarospora heufleriana from the Ozarks collected on dolomite by the Elisabeth Lay 97-0250 (hb. Lay) has a similiar powdery pruina with whitish hue, but its thallus is definitely dispersed and thicker.