A new species of Lecanora s.l. (Lecanoraceae) has been found in several localities on the coasts of the Armorican Massif (Brittany, Normandy, Channel Islands), only on siliceous rocks in the supralittoral zone. It is particularly distinguished by its well-developed thallus reacting C orange in the cortex and containing chlorinated xanthones identified by HPLC-DAD-MS. Placed in the new genus Myriolecis, it is described in this study as Myriolecis massei sp. nov. and is compared to similar maritime species containing chloroxanthones. A key is given to European maritime species of Myriolecis reacting C .