Brachysira hannae

Valves are rhombic with straight margins and rounded apices. Striae are radiate throughout valve. The central area is asymmetrically rounded and axial area is narrow and straight. Striae at the center valve margin are short, and not interrupted. The raphe is positioned within two longitudinal ribs on the external valve face. The ribs terminate at the edge of the central area and the apices. Raphe is straight and filiform. The proximal fissures are undifferentiated and the distal fissures are T-shaped. Areolae are variable in shape, mostly elongate with respect to the transapical axis. The orientation and irregular size of the areolae creates a series of wavy hyaline areas. Small papillae (visible in SEM) may be positioned on rows between the striae. A thickened rib is present along the valve margin and apex, separating the valve face from the mantle. Striae on the mantle are uniseriate and aligned with striae on the valve face.

In SEM, the internal proximal raphe fissures terminate at a small helictoglossa. Areolae are covered internally with flat, well-defined hymenes. Externally, a well-defined, thick rib defines the valve face / mantle interface. The mantle contains a single row of elongated pores, each of which is covered internally by a hymen.