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Abstract
The type material of A. islandica from Botanical Museum of Copenhagen collection was studied with the light microscopy. The seasonal variability of some characteristics (height, diameter of the valves and areolar density in 100 μm2) was analyzed. The type diagnosis of the species (diameter up to 25.5 μm, height of the valve mantle up to 28 μm, the number of areola rows between 9 to 16 in 10 urn, the number of areolae in a row, 9 to 20 in 10 μm) was enlarged and supplemented with data on the number and location of rimoportulae. The latter are located close to the ringleist, in the middle of the mantle and on the spherical valves of auxospores. The total number of them on the valves ranges from 1 to 4. The outer opening of the rimoportulae substitute for areolae in the row.