The structure of normal (A–E) and abnormal (D–K) F. radians valves strain A280 (SEM). A – an overall view; B – an axial area, inside view; C – an axial area, outside view; D – valve apex with a rimoportula (arrow), outside view; E – valve apex with a rimoportula (arrow), inside view; F – a curved valve; G – local displacement of the axial area; H – an altered shape of the valve apex, rimoportula is marked with a white arrow, non-formed or overgrown stria is marked with a black arrow; I – a displacement of the rimoportula; J – misalignment of the rows of areolae; K – merged areolae within a stria. Scale bars: A – 50 µm; B, C, G – 5 µm; D, E, H, I, J – 2 µm; F – 10 µm; K – 1 µm.