Thallus: irregular to orbicular, thin, rarely exceeding 150 µm thick, up to 3 cm across or larger because of confluent thalli lobes: loosely adnate, up to 1 mm wide, rarely broader, about the same in length; lobe tips ascending and usually crenulate, with small protrusions along the margin becoming isidia-like and eventually forming soredia, eciliate upper surface: gray to cream colored, rarely with a pruina, sorediate soredia: in marginal soralia, well developed in inner parts, particularly at the lobe angles upper cortex: paraplectenchymatous medulla: white lower cortex: paraplectenchymatous lower surface: whitish with a few pale rhizines Apothecia: sometimes present, up to 2 mm diam.; margins: with sorediate; disc: pale brown to brown, not pruinose ascospores: brown, 1-septate, Pachysporaria-type, (18-) 20-25 (-27) x (8-) 9-11 (-13) µm Pycnidia: rare Spot tests: upper cortex K+ yellow, C-, KC-, P+ yellow; medulla K-, C-, KC-, P- Secondary metabolite: upper cortex with atranorin. Substrate and ecology: at the only known locality growing on trees at 500 m World distribution: South America, East Africa and SW North America Sonoran distribution: only one locality in Baja California Sur in the Sierra de La Laguna. Notes: The thin often undulating thallus, the K- medulla and the paraplectenchymatous lower cortex are the main characters. Physcia crispa may have some similarities to P. undulata but in the latter species the medulla reacts K+ yellow and the lower cortex is prosoplectenchymatous.