Thallus thin and effuse, creamy-white to grey, composed of dispersed areoles, 0.5-1.5 mm across, on a dispersed black prothallus; medulla I-. Photobiont chlorococcoid; cells 9-12 Ám. diam.
Apothecia black, lecideine, 0.7-1.2 mm diam. ▒immersed in the thallus; disc concave; proper margin thick (0.2 mm), raised and persistent; in section thick and carbonaceous, K+ magenta solution at edge. Hymenium c. 170 Ám tall, paraphyses slender, branched and anastomosing, not swollen or pigmented at tips; epihymenium dilute brownish; subhymenium hyaline 45-50 Ám tall. Asci Porpidia-type; ascospores simple, halonate, 20-24-28 x 9-10-12.5 Ám. Hypothecium brown.
Conidiomata not observed.
Chemistry: Stictic acid by TLC.
Distribution and Ecology: Known from the type and one other collection on siliceous rocks on Isle Madre de Dios in SW Chile. No other identifiable lichen species are present on the collections.
Remarks: The thick carbonaceous margin and concave disc give this species a distinctive appearance, but the massively developed exciple is possibly an environmental modification and may not be a good species-level character. The exciple structure and chemistry suggest this species belongs to the P. macrocarpa-group (Haplocarpon M. Choisy).
Porpidia vulcanoides is unusual in the genus because of its large ascospores. The northern hemisphere species P. superba differs in having a thick, bullate, white thallus, whereas P. stephanodes from Iles Kerguelen has an even taller hymenia (170-230 Ám), larger ascospores (35-60 Ám) and an exciple having a pale brown medulla (similar to that of P. macrocarpa (DC.) Hertel & A.J. Schwab).