The study of archive materials has made it possible to specify the species composition of the genus Thalassiosira. In the Danube region of the Black Sea, a representative of Centrophyceae (Thalassiosira duostra Pienaar), which is new for the flora of Ukraine, was discovered. In the analyzed material, the number of central fultoportulae was greater than in the original description, and they are arranged in a circular pattern. However, all quantitative (valve diameter, number of central and marginal fultoportulae) and qualitative (areolae shape and their arrangement on the valve face) characteristics correspond to the published data. According to the general valve outline, the structure and arrangement of central and marginal fultoportulae as well as quantitative characteristics, T. duostra is very similar to Сonticribra weissflogii and Thalassiosira faurii, which may have caused inaccuracies in these species' identification Bacillariophyta, Thalassiosira, morphology of frustule, scanning electron microscopy, Ukraine.
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