Psora Hoffm.
Thallus squamulose or minutely lobate, squamules 1.5-5 mm diam., foliose, heteromerous, dorsiventral, loosely or closely attached, corticate on both upper and lower surfaces, saxicolous or terricolous. Upper cortex pseudoparenchymatous, cells swollen, with or without isidia, with or without soredia or pruina. Photobiont green, Trebouxia. Lower surface corticate, with short rhizines. Apothecia biatorine, 1-2 mm diam., brownish or black. Excipulum swollen. Hymenium 80-100 µm tall. Paraphyses 3 µm thick, apices thickened (5 µm) coherent, conglutinate, not branched or anastomosing. Asci clavate, 8-spored, thin-walled with clearly developed apical tholus, tholus with amyloid ring structure. Ascospores colourless, simple, thin-walled, not halonate, globose to ellipsoid.
Psora as emended by Schneider [ Biblthca lich. 13: 1-291 (1980)] contains c. 17 species. It is included in the family Lecideaceae and is characterised by a squamulose or small-foliose thallus, the swollen, pseudoparenchymatous cells in the upper cortex, a corticate lower surface with attaching rhizines, the biatorine, immarginate apothecia, a swollen excipulum and an amyloid ring structure and tholus present in the asci. The genus is widely distributed in both temperate and arid regions of the world. One species is recorded in New Zealand.
