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Abstract
The type materials of Amphora tenerrima Aleem & Hustedt and Amphora tenuissima Hustedt have been examined with light and electron microscopy in order to amplify the data on these two small species apparently lacking ventral striae. Both species, though well characterised by light microscopy in their original descriptions, appear problematic in view of information from use of the SEM. A. tenerrima from the type material has biseriate dorsal striae and ventral striae formed by one row of cuneate shallow depressions of the basal siliceous layer, a striae shape that has not been recorded for any species of the subgenus Halamphora Cleve. On the other hand, the conspicuous puncta of the connecting bands allow a differentiation of this species from very similar taxa. A. tenuissima has uniseriate dorsal striae formed by rows of small round puncta. Ventral striae are thin slits arranged in a higher density than dorsal striae. These and other fine structural features combined with morphometrical data make them distinguishable from apparently similar taxa even under the light microscope. The features revealed by SEM for both species complete the original descriptions.
