The genus Thalassiosira (Bacillariophyceae): studies on species occurring south of the Antarctic Convergence Zone

Species of the genus Thalassiosira are abundant in the oceanic waters south of the Antarctic Convergence Zone. Species within the genus are differentiated on the basis of arrangement and density of areolae and on type, position, and arrangement of processes. All of the Antarctic species known to us can be separated on the basis of characters visible in the light microscope. A key to 20 taxa including descriptions and illustrations is given. Two species are described as new: T. frenguelliopsis and T. maculata.