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Achnanthidiaceae: Eucocconeis

Frustules lanceolate or rhombic and broad in valve view, with bluntly rounded poles; twisted; bent in girdle view.

Frustules heterovalvar and isopolar, one valve with a raphe, the other without a raphe (‘monoraphid’), bent in girdle view, with the raphe valve on the ‘inside’ of the bend; raphe valve convex in transapical section (i.e. valve face domed); pseudoraphe valve concave in transapical section (valve face depressed).

Striae uniseriate, strongly radial with long and short striae alternating at the centre, can be interrupted at the junction of valve face and mantle; areolae very small, circular to quadrangular, occluded by fine pore plates (hymenes) at their inner apertures.

Axial area narrow in both the raphe and pseudoraphe valves. Central area expanded and elliptical or rhombic in the raphe valve; very strongly expanded into a large plain area in the pseudoraphe valve.

Raphe in the midline of the valve towards the centre of the valve, sigmoid overall, because of an opposite curvature at the two poles; expanded externally at the centre to form raindrop-like endings; central internal endings offset, turned in opposite directions; no terminal fissures.

Girdle composed of several narrow bands.

One lobed chloroplast per cell.