All Taxa

Bacillariaceae: Simonsenia

Cells lie in valve or girdle view, isolated valves always in valve view. Valves bilaterally symmetrical, narrowly lanceolate or linear-lanceolate, with pointed or shortly rostrate poles.

Frustules isopolar, isovalvar, valves bilaterally symmetrical, but the raphe systems of a frustule on opposite sides (i.e. frustules with rotational = diagonal symmetry).

Striae clearly visible in LM but stria pores unresolvable (in fact the striae are biseriate or multiseriate, with alternating tiny pores visible only in EM).

Axial area extremely narrow, indistinguishable in LM.

Raphe marginal, impossible to detect in LM; central raphe endings absent. Fibulae unlike those found in Nitzschia, because they are not solid bridges of silica but hollow structures, containing an extracellular tube that passes beneath the raphe, creating a narrow passage through from the valve face to the mantle. Stria pores penetrate the tube. This structure, which can only be seen with SEM, is reminiscent of the structure of some Surirella species, though the similarity is probably a homoplasy. In LM, the narrow spaces between the fibulae, giving access from the subraphe canal to the cell lumen, are more obvious than the fibulae themselves, appearing as small dark 'arrowheads'. Central pair of fibulae no more widely separated than the others.

Girdle composed of open bands.

Two chloroplasts, positioned ‘fore and aft’ (one towards each pole) as in Nitzschia.

The species are small and easily overlooked when alive.