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Abstract
The Stephanodiscus niagarae-complex is comprised of large, planktonic, morphologically variable species with dissimilarly shaped valves (heterovalvy). The species are characterized by a set of subtle morphological features causing controversy in species distinction. In this paper we present a comparative study based on a re-investigation of the original material for the following species: S. niagarae, S. rotula, S. aegyptlacus, S. neoastraea, S. galileensis and S. agassizensis. New material from Farmoor Reservoir (England), Loch Neagh (Ireland) and a lake in the Rift Valley (East-Africa) was also analyzed. The position of the valve face fultoportulae is used as a character to differentiate two groups. In one group the position of the fultoportulae is the same on each valve face, whereas in the other group the positions are different. A third group is lacking fultoportulae. A new species, S. heterostylus is described based on material from Farmoor.
