Abstract
The frustule of Cocconeis costata var. costata, its varieties hexagona and pacifica, and C. pinnata (Bacillariophyceae), mainly collected in SE Pacific Ocean coastal and antarctic waters, have been studied. Ultrastructural information is provided for the first time for most of them. The taxa show several common features in relation with the low density and areolation of the striae (except for C. costata var. pacifica); broad and highly internally silicified interstriae; areola opening and arrangement, and location of the areola occlusion; type of valvocopula and absence of fimbriae in both valves, and presence of a transverse hyaline central area in the sternum valve (or araphid valve). Only the valvocopula of the sternum valve of C. costata var. pacifica displays fimbriae which are similar to the crenate edge of a leaf. Though only partially studied, the areola occlusion shows great uniformity on comparing both valves. The taxa differ with respect to the overall morphology and the areolar density of both...
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