Guide to Amphorotia
- Valves slightly, to stongly, asymmetric to the apical axis
- Valve symmetric to transapical axis
- Rimoportulae two per valve
- Sternum on valve face
- Valves greater than 60 µm in length
Valves of Amphorotia are large in size, generally greater than 60 µm in length. The valves are asymmetric to the apical axis and symmetric to the transapical axis. In addition, the frustules are asymmetric to the transapical plane, so that the dorsal mantle is very broad as compared to the ventral mantle. Each valve has two rimoportulae, positioned near the apices. A distinct sternum is present on the valve face. The areaolae are coarse and not well organized into striae.
In North America, this taxon is only known as a fossil form. Extant populations of Amphorotia clevei are present in large oligotrophic Scandinavian and Baltic lakes. Endemic Amphorotia lineages are found in Lake Baikal, Mongolia's Lake Hövsgöl, and in China.