The fine structure of two small Amphora species. A-tenerrima Aleem & Hustedt and A-tenuissima Hustedt

The type materials of Amphora tenerrima Aleem & Hustedt and Amphora tenuiss ima Hustedt have been examined with light and electron microscopy in order to amplify the data on these two small species apparently lacking ventral s triae. 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 s pecies of the subgenus Halamphora Cleve. On the other hand, the conspicuous puncta of the connecting bands allow a differentiation of this species fro m very similar taxa. A. tenuissima has uniseriate dorsal striae formed by r ows of small round puncta. Ventral striae are thin slits arranged in a high er density than dorsal striae. These and other fine structural features com bined with morphometrical data make them distinguishable from apparently si milar taxa even under the light microscope. The features revealed by SEM fo r both species complete the original descriptions.