Asymmetrical biraphid - Valves are linear-lanceolate with subcapitate ends. The ventral side of the central part of the valve is expanded. A stigma is present on the dorsal side of the central area. Striae are comprised of two rows of areolae (biseriate).
Asymmetrical biraphid - Valves are linear-lanceolate with subcapitate ends. The ventral side of the central valve is expanded. A stigma is present on central nodule. Striae are comprised of one row of areolae (uniseriate).
Symmetrical biraphid - Valves are broadly lanceolate with apices protracted to broadly rounded. The raphe is lateral and undulate. Within a stria, the marginal areolae are large and more coarsely arranged. The middle valve has areolae comparatively smaller and rounded. Central area has large, round isolated puncta. The smallest striae at the apices are multiseriate throughout their length.
Asymmetrical biraphid - Valves linear-lanceolate. Frustules in girdle view arched (bent). In valve view, this arch prevents resolution of the entire valve face at one level of focus. Pseudoseptum present at each pole. One valve possesses a fully developed raphe system, the other valve possesses very short raphe slits.
Epithemioid - Valves linear, with apices bent towards the ventral margin. The center is deflected slightly towards the dorsal margin and notched. The raphe is positioned in a canal. Costae are prominent, and extend across the valve face.
Rhopalodia gibberula var. musculus
Epithemioid - Valves semi-elliptical, strongly convex along the dorsal margin, concave along the ventral margin, tapering to narrowly rounded ends. Costae distinct, numbering 3-5 in 10 µm. Striae are in rows of 2-4 between the costae.
Monoraphid - Valves are linear or linear-elliptic with rounded ends. The raphe valve has a linear axial area that gradually widens into a small or almost indistinct central area. The rapheless valve has a narrow, linear axial area and no distinct central area. Striae are uniseriate and slightly radiate.
Monoraphid - Valves are linear-elliptical or elliptical with slightly drawn-out ends. The striae are slightly radiate throughout both valves and biseriate. The axial area is narrow. The central area is small, or absent. If present, it may be asymmetrical on the rapheless valve.
Monoraphid - Valves are linear or linear-elliptical with rounded ends, 3.5-4.5 µm wide, 13-18.5 µm long. Raphe valve has a large transverse elliptical central area that almost reaches the valve margins. Terminal raphe fissures are absent. Rapheless valve has a narrowly rhomboid axial area and no distinct central area. Striae are uniseriate, slightly radiate, and number 32-36 in 10 µm.
Monoraphid - Valves are linear to linear-elliptical or linear-lanceolate with rounded ends. The terminal raphe fissures are not distinct. The striae are uniseriate, with 4-5 areolae per stria, and slightly radiate throughout both valves.