抄録
A new genus, Praestephanos, is described with its type species based on Stephanodiscus suzukii. In addition, two new combinations are described: P. triporus and P. carconensis. This genus is similar to the genus Stephanodiscus, however, the pattern of the areolae changes at the level of the marginal fultoportulae but not at the level of the rimportulae or spines, and this differs from Stephanodiscus. Five clades, Stephanodiscus, Cyclostephanos, Cyclotella ocellata–Handmannia complex, Praestephanos and Discostella were detected by molecular analysis. The Discostella clade has the most basal position of these five clades. The Praestephanos clade holds a clearly basal position in relation to the Stephanodiscus and Cyclostephanos clades. However, the relationship between Praestephanos and Cyclotella ocellata–Handmannia was uncertain. Since these four clades separated with short branches in the phylogenetic tree, these four clades (excluding Discostella) may have evolved during a very short geological period.
