Collemopsidium ostrearum Taxon Atlantique-Manche forme saxicole.

       Thallus very thin and inconspicuous or formed of a thin layer of whitish calcite with a brownish prothallus in contact with other thallus. Perithecia very large, single and isolated or fused side by side in twos or threes, as if resting on the thallus, conical, sometimes including calcite crystals at their base, with one or more very deep ostioles. 8-spored asci. Spores 1-septate with one part larger than the other (ice cream cone appearance), 21-26 x 8-10 µm according to our measurements. Rare species coming from the base of a hard limestone cliff at the level of Limpets and Chthamalus.