The genus Thalassiosira: species with internal extensions of the strutted processes

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Abstract
Samples from the Antarctic and Subantarctic Zones were used for study of 3 taxa of the diatom genus Thalassiosira with circumpolar distribution. Most records were from south of the Antarctic Polar Front Zone. T. perpusilla, T. gracilis var. gracilis and T. gracilis var. expecta comb. et stat. nov. [T. expecta Van Landingham] have the longer extension of the strutted process inside the valve, a single, simple labiate process away from the margin, cribra often appearing as simple rosettes of pores and constricted or partly occluded foramina. They have a particular structure of the strutted process-modified opercula partly covering the satellite pores around the internal base of the process. In some cases, these are thickened or reduced; they appear to have the same basic structure as that found in most of the genus. Differences in these species include array and size of aerolae, fine structure of foramina and spacing of marginal strutted processes. They are compared to T. confusa, from northern waters, which has the longer extension of the strutted process on the inside of the valve but has trifultate structure of these processes and modifications of aerola structure.