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Abstract
The fine structure of Aulacoseira (Melosird) baicalensis frustules has been studied by means of scanning and transmission electron microscopy. It appears that the form of the suture spines is conservative. Areolae are of two types. Type 1 areolae are loculate and are covered internally with a velum, with an additional finely perforate membrane, which is bell-shaped. Areolae of type 2 are laminar (non-loculate) with vela of the vola and rota types and do not have fine membranes, or have only minute ones. Intermediate forms are also present. The fine structure of the copulae has been investigated: they are perforated by fine pores (20–40 nm in diameter) in regular rows.