Frustules isopolar; linear, linear-elliptical or lanceolate in valve view, with bluntly rounded or very slightly rostrate poles; rectangular with rounded corners in girdle view; girdle deep (hence cells often lie in girdle view).
Frustules isovalvar, diagonally (rotationally) symmetrical about the median valvar plane, as in most Nitzschia (i.e. the raphe systems of the two valves are diagonally opposite, rather than on the same side of the cell).
Striae uniseriate or biseriate (multiseriate in two species currently assigned to the genus but probably not belonging there [D. rainierensis and D. jamesrossensis]), parallel throughout; some ribs (interstriae) greatly thickened to form wall-like partitions (sometimes called ‘costae’) that traverse the valve from margin to margin; areolae circular.
Axial area narrow, central area absent.
Raphe slightly eccentric to marginal (and on opposite sides of the frustule in the two valves), sometimes raised on a slight, rounded keel; subtended by fibulae, which are formed where the wall-like partitions cross beneath the raphe; the bases of the fibulae are expanded in the apical direction to form flanges that restrict access to the raphe canal to elliptical holes (portulae); central raphe endings present or absent; polar raphe endings turned or hooked towards either the nearer (proximal) or farther (distal) valve margin.
Bands open; some species have ‘half-bands’ (bands that occupy only one side of the frustule; the most advalvar band(s) are modified to fit around and grip the transapical wall-like partitions.
Two plate-like chloroplasts per cell, arranged ‘fore and aft’ (one towards each end), as in Nitzschia.