Symmetrical biraphid - Valves elliptic with rostrate to rostrate-apiculate ends. The axial area is narrow. The raphe is filiform and straight to slightly undulate, with prominent, dilated external proximal raphe ends. Areolae are slightly offset from one another, producing decussate striae.
Nitzschioid - Valves are small, with rounded ends. Striae are not visible in LM. Costae are parallel, becoming curved at the apices.
Nitzschioid - Valves are small, with acute ends. Striae of some specimens are visible in LM. Costae are mostly parallel.
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.
Diatoma vulgaris var. ehrenbergii
Araphid - Valves are narrow and elongate. Apices are sub-rostrate to capitate. A single rimoportula is present near the apical pore field.
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 - Valves are elliptical. The central area is large and round. Striae are radiate throughout, becoming more strongly radiate near the apices. Striae are uniseriate. Most specimens have a single row of areolae along the longitudinal canal, but in some specimens there may be two rows of areolae.
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.
Symmetrical biraphid - Valves are elliptic to oval and up to 33 µm wide. The longitudinal canals are narrow. Striae are uniseriate and radiate throughout. The central area is large and orbicular, up to 8.5 µm wide.
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.