Achnanthidium deflexum

Valves are linear-elliptica to elliptic, with subrostrate ends. The raphe valve is concave. The raphe valve possesses a linear axial area, which widens slightly in the middle portion of the valve. The terminal raphe fissures are hooked towards the same side of the valve and are slightly expanded at the ends. Externally, the proximal raphe ends are teardrop-shaped. Internally, the proximal raphe ends curve to opposite sides. The rapheless valve is convex. It has a narrow, linear axial area widening slightly in the middle. Striae are parallel or very slightly radiate throughout both valves. Stria density of the raphe valve is 15-28 in 10 µm in the middle and 35-50 in 10 µm at the apices. Stria density of the rapheless valve is 15-26 in 10 µm in the middle and 30-35 in 10 µm at the apices. The external openings of the areolae are transapically elongated, appearing as narrow slits. Each stria in the middle part of the valve is composed of 4-5 areolae. Very short marginal striae are often present in the middle portion of the valve.

Note that the great range in stria density within these observations, as well as within the original description, is due to the strong gradient of change in stria density particularly at the ends of the valves. Furthermore, the density depends on the length measured. For example, the number of striae in 10 µm, 5 µm or 2 µm (measured in SEM images) will each result in a different measure.