Collemopsidium halodytes (Nyl.) Grube & B. D. Ryan s.l.

   Thallus superficial, translucent at first, then opaque, pale brown, brownish, smooth, more or less regular, without black ridges, sometimes, but rarely, with minute black or brown dots. Perithecia flattened, sessile, 0.1-0.4 mm, involucrellum spreading laterally, in old specimen ostiole deeply immersed lifebelt-like. Ascospores oblong, 1-septate, 13-22 x 5-9 µm according to the literature; 15-18 x 6-8 µm according to our measurements. Pycnidia  common, resembling to perithecia but smaller. Photobiont : cyanobacteria. Common (but often neglected) species found on rocks or large stones, siliceous (see: Collemopsidium halodytes forme saxicole sur roche silicatée) or limestone (see: Collemopsidium halodytes forme saxicole sur roche calcaire) of the coastal zone, more rarely on the Limpets (but see: Collemopsidium halodytes forme sur Patella which also has a thallus but which may be a different taxon).

_ This species can be distinguished from Collemopsidium elegans by the absence of warts and ridges on the thallus and its flattened perithecia.

_ Verrucaria ditmarsica s.l. may also be confused but its perithecia are hemispherical, ascospores simple and it grows higher on the foreshore.

N.B. Species taken here in its current sense but there are probably several taxa under this name.