Paulia myriocarpa  

 Thallus: umbilicate, rosette-shaped, squamules roundish in outline, up to 4.5 mm wide; 'areoles': 0.06-0.25 mm wide, margin effigurate, marginal lobules 0.1-0.4 mm long; surface: black, regularly tessellate,; Apothecia: immersed to semi-immersed, lecanorine, 1-3 per squamules, small, up to 0.2 mm; disc: blackish, flat, with persisting thalline margin; exciple: lacking; epihymenium: yellowish brown; hymenium: 80-160 µm high, hyaline, iodine reaction variable; paraphyses: distinctly septate, sparingly to richly branched and anastomosing, apical cells slightly enlarged, up to 3.5 µm wide; asci: 8-spored; ascospores: simple, hyaline, broadly ellipsoid, 11-15 x 7.5-9 µm wide; old spores: with thick (-3 µm) wall; Pycnidia: immersed, broadly pyriform, c. 0.1 mm wide; conidia: cylindrical, hyaline, c. 3.5 x 1 µm; Spot tests: all negative; Secondary metabolites: none detected.; Substrate and ecology: on volcanic, calcareous or siliceous rocks, mainly on vertical or inclined, sheltered rock faces or along seepage tracks; mainly coastal or island habitats, rarely inland; World distribution: SW North America, the neotropics and Socotra Island (Indian Ocean); Sonoran distribution: southern Baja California Sur.; Notes: This is a small but very distinct species because of its regularly tessellate thallus surface with its effigurate margin.