Thallus: entirely immersed or very sparse surface: apparently white or whitish green photobiont: a cyanobacterium, Hyella; cells: orange Perithecia: to 0.2-0.6 (-1.5) mm diam. [on soft chalk, half-immersed by erosion of the substratum, to 1 mm on limestone and then subsessile (even on hard rock)]; black; ostioles not prominent involucrellum: brown-black, extending part way down over the exciple and spreading; exciple: hyaline to pale brown, to 200 µm diam. hamathecium: much-branched paraphysoid network, somewhat persistent asci: subcylindrical to cylindrical, c. 8-spored ascospores: narrowly ovoid, hyaline, 1-septate, cells unequal in size, 12-20 (-24) x 5-7 (-10) µm Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: on calcareous rocks amongst seaweeds in the sublittoral zone, especially ridges uncolonized by barnacles, and on shells (barnacles and limpets) World distribution: Europe, west coast in North America, and New Zealand Sonoran distribution: coastal California [reported from central part; may also occur in southern part]. Notes: In many treatments prior to 1992 this species was treated as a synonym of C. halodytes. Santesson (1992) distinguishes the species on the basis of the immersed thallus. However, C. sublitoralis has a whitish thallus and larger, more prominent perithecia. This species is not yet reported from the area, but it is likely to occur.