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Abstract
A new raphid diatom genus, Lunella, is described. The new genus is probably closely related to Parlibellus E.J. Cox, but differs from it by a “catenuloid” cell construction. The valve face of Lunella is asymmetrical about the apical plane: it has a straight ventral margin, a convex dorsal margin, and an eccentric raphe situated near the junction of the valve face and the ventral mantle. Furthermore, the taxa belonging to Parlibellus typically have a wide girdle region with numerous girdle bands and often a strongly convex valve face, whereas Lunella possesses fewer girdle bands and a flat valve face. The type species of the genus, Lunella bisecta sp. nov., is described from brackish water (5 %°) in the northern Baltic Sea at Forsmark in Sweden. Highest abundance of this species was found in late winter (February-March), when it was growing in an epilithic mucilage matrix together with Nitzschia inconspicua Grunow and other diatoms in water of 0°C. Under the light microscope L bisecta resembles Catenula adhaerens (Mereschk.) Mereschk., but it is shorter with less acute apices, the helictoglossae are less pronounced and the striae are not visible. Scanning electron microscopy shows that the fine structure of L bisecta is fundamentally different from that of C. adhaerens.