Abstract
Several freshwater species of Synedra Ehrenb. have been investigated using light and electron microscopy. The morphology of the vegetative valve and girdle region is discussed and an evaluation of the taxonomic position of Fragilaria ungeriana Grun. is made in the light of new evidence. Homologies have been identified that characterise a small group of Synedra (Eusynedra sensu Round 1979). F. ungeriana is more correctly placed within this genus, with the consequence that linking spines are no longer sufficient to characterise the genus Fragilaria. Some aspects of the higher classification of the araphid diatoms are explored and it is proposed that the definitions of Synedra and Fragilaria made by Lange-Bertalot (1980) and Round (1984) are both inadequate.