The new genus and species Isalonactis madagascariensis is characterized by a crustose, non-corticate, often sorediate thallus containing psoromic acid, tiny white pruinose ascomata with a thalline margin, an inconspicuous excipulum, a pale brown hypothecium, 3-septate hyaline ascospores and curved filiform conidia. Phylogenetic analyses using nuLSU and RPB2 sequences place Isalonactis in the Roccellaceae, close to the genera Lecanactis and Chiodecton. The new species was collected on sheltered siliceous rocks in the dry landscape of the Isalo Massif (S Madagascar). Dermatiscum thunbergii is newly recorded from Madagascar.
