Planothidium bilensis sp. nov. has unusual contacting valve with two lensoids, situated opposite to each other, and diagonal position of sternum. Both lensoids consist per one siliceous lense-like db-element and plain-convex lens-like hollow next to it. The main types and some kinds of the lensoids are discussed and new terms plain-convex lensoid (pc-lensoid) and biconvex lensoid (bc-lensoid) are suggested for morphological description of the diatom frustule. Variable morphology of the lensoids corresponds to species rank of taxonomy and may serve as a clear differentiating character for morphologically relative species. New species was found in the small pond in the landscape reserve “Island Zhukiv”, right bank valley of the Dnipro River, South boundary of Kyiv.

Keywords: diatoms, functional morphology, db-element, df-morph, lensoid, ecology, species typification, holotype

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