Symmetrical biraphid - Valves are linear, with rounded ends. Vavles are 4.5-5.6 µm wide. The striae cannot be resolved with under LM, but the upper surface of the valve is covered by a conopeum, which has small pores arranged into fascicles that look like striae or "wrinkles" along the raphe under LM. The axial area is very narrow and there is a small elliptic central area. At each apex there are two openings between conopeum and the valve margin. The proximal raphe ends are very close together and slightly bent to the primary side of the valve.
Symmetrical biraphid - Valves are broadly lanceolate with rounded apices. A lyre-shaped unornamented area bisects the striae on either side of the axial area, connecting to form the central area.
Araphid - Frustules are linear with rostrate to subcapitate apices. The central margin of valves is expanded to one side. The striae are broadly spaced, that is, costae are wide. The striae are interrupted by an expanded central area usually extending to one, or less commonly, to both sides of the valve. The striae are parallel, to slightly radiate near the valve ends. In some specimens, the striae continue through the central area. One rimoportula is present on each valve, positioned near the valve terminus. The rimoportula is oriented parallel, or slightly tilted, with respect to the transapical axis.
Araphid - Valves are cruciform and up to 20 µm long. Striae number 34 10 µm at the center to 40 10 µm at the ends.
Araphid - Valves are linear with slightly rostrate to bluntly rounded apices. The sternum is very narrow. A rimoportula are present at one or both poles, which is visible in many specimens in LM. Short linking spines are most visible in frustules in girdle view.
Araphid - Valves are linear to lanceolate with rostrate, broadly rounded ends. In girdle view, frustules are rectangular with undulate ends. Striae are composed of round areolae. Spines are positioned on the costae. Two distinct, simple porefields are present at each apex, located on the valve mantle near the junction with the valve face.
Symmetrical biraphid - Transapically expanded areolae are present at the central area. The distal ends of the longitudinal ribs run parallel to the linear helictoglossa, enclosing roughly half of its length.
Symmetrical biraphid - The porte-crayon is robust and slightly curved. The central nodule is elongate, especially in larger specimens. Isolated poroids are often present in the central area and near the distal raphe ends. The longitudinal ribs and raphe are slightly curved.
Symmetrical biraphid - Unlike most species of Frustulia, F. asiatica has longitudinal ribs that are incomplete at the valve center. The valves are linear-lanceolate and not at all rhomboid. The valves are linear-lanceolate rather than rhomboid. The external raphe ends are forked, forming a small Y shape. The central area is oval. Small, irregular poroids are present between the raphe and striae.
Symmetrical biraphid - The longitudinal ribs of F. capitata are complete only on the side of the valve, the same side to which the proximal raphe ends are deflected. The valves are linear-lanceolate with capitate to subcapitate apices. The external proximal raphe ends are unilaterally deflected.
Frustulia rhomboides var. crassinervia
Navicula rhomboides var. crassinervia
Symmetrical biraphid - The valves of F. crassinervia are rhomboid in shape, with undulate margins and moderately protracted apices. The porte-crayon is small. At the valve center, the longitudinal ribs are constricted, but only slightly.
Symmetrical biraphid - Unlike other species of Frustulia, F. creuzburgensis lacks internal longitudinal ribs. The external proximal raphe ends are unilaterally deflected, while the distal raphe ends are straight. The distal raphe ends terminate in a transverse depression that contains poroids. The helictoglossae are prominent in LM.
Symmetrical biraphid - The striae are not circumradiate at the valve apices. The longitudinal ribs and raphe are noticeably curved. The longitudinal ribs are moderately or slightly constricted at the central nodule. Longitudinal striae are present but are very wavy. The transapical striae are generally parallel.
Symmetrical biraphid - Frustulia krammeri has striae that do not circumradiate the poles.The valves are strongly rhomboid, with a slight constriction at the valve apices. The longitudinal ribs and raphe curve slightly in the same direction. Near the apices, the striae become moderately wavy and then convergent at the apices.
Symmetrical biraphid - The internal, longitudinal ribs of F. latita are incomplete in the center of the valve. The central striae are variable in length, creating an asymmetric central area. The valve apices are very slightly rostrate. Externally, the proximal raphe ends are deflected unilaterally.
Symmetrical biraphid - The longitudinal ribs of F. neomundana are complete on one the side of the valve, the same side to which the proximal raphe ends are deflected. The valves are linear-lanceolate with rostrate apices. The external proximal raphe ends are unilaterally deflected.
Symmetrical biraphid - The valve apices are protracted and moderately rounded. The valve margins are moderately undulate.
Symmetrical biraphid - The valves of F. rexii are strongly rhomboid, with a very slightly constriction at the valve apices. A group of external poroids is present in the central area. Near the apices the longitudinal ribs are abruptly wider, giving the longitudinal ribs a unique appearance. Striae circumradiate the apices.
Symmetrical biraphid - The degree of rhomboid valve shape is variable within populations, in direct proportion to valve size. The apices are slightly constricted and a relatively small porte-crayon is present.
Symmetrical biraphid - The longitudinal ribs in Frustulia soror are complete only on one side of the valve, the side of the valve to which the proximal raphe ends are deflected. Valves are linear-lanceolate with slightly rostrate apices. The external proximal raphe ends are unilaterally deflected.
Symmetrical biraphid - The valves are linear-lanceolate-rhomboid in shape with rostrate apices. The striae are slightly radiate at the center of the valve, parallel or slightly convergent near the apices, and convergent at the apices. The longitudinal ribs are discontinuous at the center of the valve and do not fuse with the central nodule. The central area is ovoid in shape. The distal, external raphe fissures form a small Y-shape, usually visible only with SEM.