STUDIES ON THE GENERA GYROSIGMA AND PLEUROSIGMA (BACILLARIOPHYCEAE) -GYROSIGMA-ACUMINATUM (KUTZING) RABENHORST, G-SPENCERI (QUEKETT) GRIFFITH, AND G-RAUTENBACHIAE CHOLNOKY

Gyrosigma spenceri (Quekett) Griffith & Henfrey is a synonym of G. acu minatum (Kutzing) Rabenhorst. Type specimens do not show any morpholog ical discontinuities. In particular, they do not differ in the longitu dinal/transverse stria density ratio, which has previously been claime d as a decisive criterion. The marine-brackish ecology claimed for ''C yrosigma spenceri'' is excluded by its type material. This synonymy of two of the most commonly identified Gyrosigma taxa makes verification of a mass of published data on distribution and ecology necessary. Th e numerous ''varieties'' of ''G. spenceri'' all bear invalid names. Th e organisms called ''Gyrosigma spenceri'' sensu Hustedt (1930), sensu Van der Werff & Huls (1957-1976) or sensu Krammer & Lange-Bertlot (198 6) are incompatible with the type and as yet undefinable. Gyrosigma ra utenbachiae Cholnoky resembles G. acuminatum but shows specific differ ences from that organism. A practical definition of the section Acumin ati sensu Peragallo (1891) would include organisms with a longitudinal /transverse stria density ratio from ca. 1.00 to 0.85.