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Achnanthidiaceae: Planothidium

Frustules heterovalvar, isopolar, bilaterally symmetrical. Raphe valve slightly concave, rapheless valve correspondingly slightly convex; in girdle view, frustules arched.

Cells usually lie in valve view, Valve outline elliptical, elliptical-lanceolate or lanceolate, with a shallow mantle; apices rounded, rostrate, capitate or apiculate.

Striae bi- or multiseriate, radial or (more rarely) parallel, usually interrupted on one side of the centre in the rapheless valve by a horseshoe-shaped plain area (the ‘hufeisenförmige Fleck’, which is hollow internally and may be partly covered by a hood (canopy) visible in LM as a curved line crossing the space.

Axial area narrow and linear in the raphe valve, sometimes wider in the rapheless valve.

Central area indistinguishable from the axial area, rounded or transversely elongate; central area asymmetrical on the rapheless valve when the horseshoe-shaped area is present.

Raphe straight, external central endings expanded, internal central endings deflected in opposite directions (noncoaxial, offset); external terminal raphe endings deflected to one side.

Girdle bands open.

A single chloroplast, with a single pyrenoid located below the horseshoe-shaped area.

Attached to the substratum via the valve face of the raphe valve.