Abstract
In this study, based on the results of molecular analysis of SSU rRNA and rbcL sequences, we propose the descriptions of two new families in the order Cymbellales. Molecular data demonstrates that diatoms of the genus Encyonema constitute an independent monophyletic clade, which represents a new family described herein - Encyonemataceae fam. nov. Another family introduced in this article, with regards to molecular data, is Witkowskiaceae fam. nov. In our research, it is comprised of three closely related genera - Geissleria, Paraplaconeis and Witkowskia. The results of performed molecular investigation are supported by morphological analysis. Morphological diagnoses of the new families are based on a combination of features: type of valve symmetry, raphe structure, presence and position of stigmata or stigmoids, number and location of apical pore fields and morphology of chloroplast. In addition, we discuss the phylogeny of selected genera of the order Cymbellales. Based on their morphology, 3 and 5 genera are assigned with provisionary position in Encyonemataceae fam. nov. and Witkowskiaceae fam. nov., respectively.