Abstract
Material chosen as the lectotype of Amphora tumida Hustedt by Simonsen was examined with light and scanning electron microscopy. Based on this analysis a more comprehensive concept of this taxon was established and the description emended. A comparison with the morphologically similar A. subacutiuscula Schoeman and A. acutiuscula Kützing sensu Archibald strongly suggests that A. tumida is present in South African waters, in addition to its confirmed records from Venezuela and Argentina.