Frustules slightly bent in girdle view with a raphe system only on one valve (monoraphid); linear, lanceolate or elliptical in valve view, occasionally with broadly rostrate apices; girdle narrow, so that cells most often lie in valve view.
Frustules heterovalvar, isopolar, bilaterally symmetrical (but the raphe is sometimes oblique to the apical axis); bent in girdle view, with the raphe valve on the ‘outside’ of the bend; the raphe valve and pseudoraphe valve are ± similarly patterned.
Striae uniseriate, parallel to slightly radial, fine (relative to the small size of the valves in this genus); areolae small, circular occluded by fine pore plates (hymenes) at their inner apertures.
Axial area narrow and linear, or broader and lanceolate. Central area expanded, sometimes unilaterally, rounded, oval, or an acute-angled fascia or subfascia.
Raphe straight or slightly sigmoid; external central endings simple; internal central endings offset, deflected in opposite directions; terminal fissures absent or turned to opposite sides.
Girdle structure unknown.
A single chloroplast per cell, lying against one side of the girdle and one valve.
Diatom Flora of Britain and Ireland
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Psammothidium
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Achnanthidiaceae: Psammothidium