Thallus: arachnoid, chalky white
Apothecial pigments: brown K+ chocolate-brown
Paraphyses: sparingly branched and anastomosing
Asci: cylindrical
Spores: 3-septate; (14-)17-21(-24) x 5.5-6 µm.
Conidia: bacilliform, 4-6 x 0.8 µm.

Distribution and Ecology:
Falkland Islands. Five collections; four from Kidney Island, one from nearby Port William. On coastal siliceous rock. A further collection from Fox Bay, West Falkland is apparently the same species but is growing in close association with Verrucaria maura.

Similar to the Northern Hemisphere Arthonia calcarea (syn. O. calcarea) but that species has K+ green apothecial pigments and ±clavate asci. Also similar to O. diaphoriza from New Zealand and Macquarie Island, but that species (type of O. macquariensis) has a smooth, cracked areolate thallus.

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Thallus and apothecia

Ascus & thecium

Collection with V. maura

Thallus and apothecia

Thallus and apothecia

Ascus and thecium