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Abstract
The morphology of some very small Stephanodiscus and Cyclostephanos species from Canada, USA and Europe has been studied by light and electron microscopy. The possible influence of the environment on the variation of those characters often used in the diagnosis of species is discussed and an attempt is made to improve understanding of the features used to characterize such species as Cyclostephanos tholiformis Stoermer, Håkansson & Theriot, C. delicatus (Genkal) comb. nov. and S. cf. minutulus (Kütz.) Cieve & Mölier. Two new species are described: Stephanodiscus binatus Håkansson & Kling sp. nov. and S. nipigonensis Kling & Håkansson sp. nov. Similarities and differences to original material are further shown and discussed.
