Thallus: crustose but sometimes slightly bullate, effuse, 0.2-0.5 mm wide surface: varying from yellowish white to white to grayish white, smooth, epruinose to slightly pruinose; soredia and isidia: not seen cortex: 20-40 µm thick Ascomata: numerous, circular or elongated in outline, immersed or subimmersed, 0.4-0.8 mm diam.; exciple: usually immersed and coinciding with the thallus but often slightly elevated and distinguished from the thallus by a brownish marginal surface; hymenium: 50-140 µm high; paraphysoids: sparsely branched, parallel, hyaline, c. 1 µm in diam. asci: clavate, 70-120 x 15 µm, 8-spored ascospores: fusiform, hyaline, 3-septate, 21-27 x 5-6 µm Spot tests: thallus K-, C+ red (and KC+ red) in rimulae and thalline margin otherwise C-, KC-, P-; medulla K-, C+ red, KC+ red, P- Secondary metabolites: erythrin, lecanoric acid, orsellinic acid and two unknown substances. Substrate and ecology: growing on vertical or overhang rocks and cliffs near the sea; occasionally abundant with individuals forming large mosaic patterns World and Sonoran distribution: Baja California Sur, Baja California and Sinaloa on the mainland. Notes: Dirina mexicana falls witDirina mexicana falls within the group with immersed ascocarps: D. insulana, D. immersa and D. cretacea (Tehler 1983). It is similar to the European species Dirina insulana and D. immersa but is distinguished from those two by the yellowish tinge of the thallus. However, this character is variable and sometimes the thallus is merely white or white-grayish. Another characteristic of Dirina mexicana is the often brownish color of the thin, slightly elevated thalline margin. Dirina mexicana is partly sympatric with the other two Dirina species of the region, D. paradoxa and D. catalinariae, both of which have conspicuously sessile apothecia with constricted bases in contrast to D. mexicana with its immersed apothecia.