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Abstract
The centric diatom Coscinodiscus bipartitus Rattray 1889 is characterised by the presence around the girdle of an unusual mucilaginous matrix which expands into a variable number (0–16) of radiating lobes. The fine structure of this neritic, warm-water diatom is described by scanning electron microscopical observations on plankton from the Gulf of Carpentaria (Australia), the Fly River (Papua New Guinea) and sediments from Singapore and southern Japan. The presence of many marginal fultoportulae and two radially orientated marginal rimoportulae, together with the incidence of chambered areolae with internal crihrum and external foramen, justify the transfer of this species to the genus Thalassiosira Cleve. The close relationship between T. bipartite (Rattray) comb. nav. and the genus Planktoniella Schütt is discussed.