Thallus: crustose, usually totally leprose, forming +regular rosettes up to one cm in diam. when young, later sometimes becoming confluent with other thalli forming irregular patches to several cm in diam., indeterminate to weakly determinate, well fastened to the substrate, not forming true lobes, but often with obscure sublobes (as the soredia are often clustered towards the margin), which are often marginally reflexed upper surface: whitish to bluish gray, rough, composed of soredia soredia: variably sized, mostly coarse, up to 150(-200) µm wide, sometimes largest along thallus margin, distinctly pruinose, rather soft, without or often with short projecting hyphae or sometimes on marginal soredia, with up to 0.2 mm long projecting hyphae medulla: not evident, but sometimes with white patches containing masses of crystals not dissolving in K and superficially resembling a medulla in thick parts of the thallus lower surface: not evident, without a distinct tomentum. Note: Several chemotypes are here attributed to L. caesioalba (see also Leuckert et al. 1995, Orange 1999). However, it is possible that they may prove to represent distinct taxa or that they are better assigned to other species. Accordingly, their Sonoran occurrences are treated here individually. See also L. borealis.