Nitzschioid - Valves are isopolar, linear to linear-lanceolate, sometimes with slightly undulate margins, tapering to narrow apices. The keel is located in the center of the valve.
Symmetrical biraphid - Valves are lanceolate with slightly protracted apices. The axial area is wide and lanceolate. Areolae within striae are easily observed under LM. A longitudinal row of areolae is adjacent to the raphe slits on the primary valve side. A conopeum is visible as a longitudinal line crossing the striae.
Symmetrical biraphid - Boreozonacola hustedtii has triundulate valves with a circular central area. Striae are radiate in the middle.
Symmetrical biraphid - Boreozonacola olympica has linear valves with parallel or slightly concave sides. Striae are parallel in the center part of the valve. The central area is small and elliptical.
Symmetrical biraphid - Brachysira arctoborealis has large rhombic valves up to 48 µm long and 12 µm wide. Striae number 24-26 in 10 µm.
Anomoeoneis serians var. brachysira
Symmetrical biraphid - Valves are rhombic-lanceolate to elliptical-lanceolate in valve outline. Striae are weakly radiate across the entire valve face. The raphe is positioned within two external ribs on the valve face. The ribs terminate at the edge of the central area and the apices. The areolae are irregular in shape and spacing, creating a “wave-like” appearance of the striae. The orientation and size of areolae, creates a series of wavy longitudinal hyaline zones. Central area is small and circular to slightly oval. Valves may vary in the development of external ribs.
Symmetrical biraphid - Valves are rhombic with straight margins. The valve length is two to three times the valve width. Within each stria, the orientation and irregular size of the areolae creates a wavy appearance of the striae. Striae at the center valve margin are short, and not interrupted.
Symmetrical biraphid - Valves are lanceolate to rhombic-lanceolate with protracted, subcapitate to capitate apices. The central striae are radiate at the center, to parallel at the apices. Areolae are variable in shape and arranged in a series of two to three longitudinal hyaline zones. The central area is small and asymmetrically rounded.
Symmetrical biraphid - Valves are narrowly lanceolate, with a length to width ratio of approximately 5:1. The striae are strongly radiate at the center, becoming parallel at the apices. Irregular striae are present near the central area. Areolae are distinctly rectangular and range 2-4 per stria. The distal raphe is distinctly unilaterally curved.
Symmetrical biraphid - Brachysira ocalanensis has lanceolate to linear-lanceolate and occasionally heteropolar valves. Apices are narrowly rounded, subrostrate, or weakly subcapitate. The central area is a narrow ellipse with the long dimension oriented apically. Striae number more than 31 in 10 µm.
Symmetrical biraphid - Valves are lanceolate. The striae are slightly radiate to parallel. The axial area is linear and distinct. The central area is asymmetrically rounded. The raphe is positioned between two external longitudinal ribs. The raphe is straight with undifferentiated proximal fissures and deflexed T-shaped distal fissures. Areolae are rectanglular and elongated in the transapical axis.
Symmetrical biraphid - Valves are lanceolate with unprotracted apices. The central area is apically elongate, with a median constriction, a feature most pronounced in the largest valves. Striae number 26-30 in 10 µm.
Symmetrical biraphid - Brachysira vitrea has broadly elliptic-lanceolate valves with capitate apices. Striae number 32-36 in 10 µm.
Symmetrical biraphid - Valves are linear, with bluntly rounded apices. The central area is large. A single, wavy longitudinal line is evident on each side of the raphe. Striae number 30-32 in 10 µm.
Asymmetrical biraphid - Brebissonia lanceolata has large lanceolate to lanceolate-elliptic valves, with slightly protracted, broadly rounded to attenuate ends. Striae are parallel in the center of the valve, becoming radiate toward the ends. The elongate central nodule results in a large gap between proximal raphe ends.