On the thalline lobes of Pyxine subdnerea there are delimited patches of pruina consisting of bipyramidal prisms, bipyramids and druses of weddellite, i.e. calcium oxalate dehydrate (COD), as revealed by X-ray analysis and SEM observations. The disappearance of the crystals from older thalline areas is preceded by an increase in the frequency of corrosion figures, in the form of etch square pits, and a decrease in the number of crystals occurring on the thalline surface. These features follow a dissolution gradient from the lobe apex towards the proximal edges of the patches. The corrosion figures are linked to the thermodynamic metastability of COD. Partial dissolution of the crystals, performed in water and in EDTA, has confirmed this latter assumption. It is thought that the disappearance of pruina, which interferes with the transmission of light to the algal layer, may allow the formation of soredia in mature thalline portions.