Guide to Amphora

Valves are asymmetric to the apical axis and symmetric to transapical axis. On the dorsal margin, the valve mantle is deeper than on the ventral margin. As a result, the frustule is wedge-shaped, similar to a section of an orange. This wedge shape prevents complete focus with a microscope in one focal plane. In most species of Amphora, there is an abrupt differentiation between the valve face and mantle by a distinct marginal ridge. The raphe is moderately to strongly eccentric, and positioned on the ventral side of the valve face. The raphe may be straight, arched or slightly sigmoid.

According to Levkov (S. Spaulding, personal communication, 2011), the proximal raphe ends terminate internally with narrow, elongated rectelevata (SEM feature). Usually, the striae on the dorsal margin are interrupted and a dorsal fascia (hyaline area) is present. The striae on ventral margin are short and may be composed of a single areola. Depending on the orientation of the valve, the striae on the ventral margin may be difficult to discern. Valves lack stigmata. Terminal nodules are indistinct.

Species within the genus Amphora reach their greatest diversity in marine habitats. Of the freshwater species, A. ovalis is one of the widely distributed species. Other species, such as A. calumetica are restricted in distribution and is known only from the Laurentian Great Lakes (Edlund and Stoermer 1999).