Brachysira brebissonii

Valves are small, rhombic-lanceolate to elliptical-lanceolate with narrow rounded apices. Striae are weakly radiate from center to apex. The axial area is narrow and straight. The raphe is positioned within two ribs that lie on the external face of the valve. The ribs terminate at the edge of the central area and the apices. The raphe is filiform and straight, with expanded drop-like proximal fissures and T-shaped distal fissures. Areolae are typically elongate. The striae are interrupted by hyaline areas, creating a “wave-like” appearance of the striae. Central area is small, circular to slightly oval. Small papillae (silica nodules) are often scattered across the external valve face, visible in SEM. 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 are unilaterally deflected. The distal fissures are bounded by small helictoglossa. Areolae are covered internally with flat hymenes.