Symmetrical biraphid - Valves elliptical with rostrate to rostrate-apiculate apicies. Axial area narrow, expanded at the center to form a circular central area. The raphe is filiform and straight to slightly undulate, with prominent, dilated external proximal raphe ends. Areolae from neighboring striae slightly offset, producing a quincunx pattern.
Nitzschioid - Valves lanceolate to elliptic-lanceolate, with protracted apices. The raphe is distinct, positioned off center and contained within a canal. Costae are distinct and number 5-7 in 10 µm.
Symmetrical biraphid - Valves elliptical with apices protracted in larger specimens. There is a rounded central nodule visible when focusing on the interior of the valve. Striae are radiate, punctate and of variable length.
Araphid - Valves elliptical-lanceolate with broadly rounded apices. Transapical striae and costae visible in the light microscope.
Diatoma tenue var. moniliforme
Araphid - Valves elliptical to elliptical-lanceolate with rounded to subcapitate apices. Transapical ribs clearly visible under LM, striae not visible under LM.
Diatoma elongatum var. tenuis
Araphid - Valves are linear with capitate apices. Transapical ribs number 6-11 in 10 µm. Striae are not distinguishable under LM. One rimoportula is positioned at one valve apex.
Araphid - Valves elliptical to elliptical-lanceolate with broadly rounded subrostrate apices. Transapical costae visible under LM, striae not visible under LM.
Symmetrical biraphid - Valves linear, with rounded apices. The axial area is wide. A septum is present extending the entire length of the valve, and has three large openings, the largest in the center, smaller two at the ends. The proximal raphe ends are relatively distant from one another; they are widely spaced.
Asymmetrical biraphid - The valves are large and robust. The headpole is distinctly capitate, while the footpole may be capitate or not. The number of stigmata is variable, but depends on valve size. A porefield is present at the footpole. The distal raphe is deflected at the footpole, rather than bisecting it.
Symmetrical biraphid - Diploneis finnica has large elliptical valves with biseriate striae and a small central area. The longitudinal canals are wide and lanceolate, 1/3 to 1/2 the width of the valve at the center.
Araphid - Valves are linear with broad, rounded ends; the smallest valves within the size series are nearly circular. A single row of pores is present along the valve margin. Frustules are joined into filaments. The valve mantle is extended to form apical prows, a feature visible in girdle view. Under SEM, it is possible to view an open cingulum. This species is currently known only from wet walls on the island of Hawai’i.
Centric - Valves are extremely small, less than 10 µm in diameter. The central valve may be flat, concave or convex. Ornamentation in the central valve may be absent, or there may be a stellate ornamentation. Marginal fultoportulae are evident in LM. These fultoportulae may possess externally projecting tubes and/or hoods.
Centric - Valves are either concave or convex in the center. Central striae form a stellate pattern, although the central striae may be absent or composed of ghost striae.The marginal striae are of equal length.
Araphid - Valves linear with capitate ends and less than 3 µm wide. Costae number 11-17 in 10 µm. Striae number 20-26 in 10 µm.