In these genera, valve surface ornamentation is radially symmetric around a central point and the valve outline is circular. A few species, thus far not seen in the Great Lakes, are elliptical. In these genera, valve surface ornamentation is bilaterally symmetrical to a central thickening called the central sternum. In a few species the central axis is bent. These genera are also bilaterally symmetrical. One valve has a central sternum. The other has fissures through th ecentral sternum called a raphe. The raphe has two branches separated by a blank area. These genera also have a raphe on both valves, but are asymmetric to either the longitudinal or transverse axis. An apical pore field may be visible on one or both ends of the valve. These genera have raphes on both valves, raised above the valve surface on a structure called a keel. The keel may be on the midline of the valve or displaced toward the valve margin.

In these genera, valve surface ornamentation is radially symmetric around a central point, but the valve outline is bipolar or multipolar. These genera also have a raphe on both valves, but it is reduced to a short structure on each end of the valve. Longitudinal axis is usually bent. The convex margin of some species is elaborately scalloped. These genera have a raphe on both valves and are symmetrical to both the longitudinal and transverse axis of the valve. This is a very large and diverse group. These genera have a raphe on both valves which is borne on a specialized tube-like structure called a canal. The canal raphe is usually displaced from the midline of the valve. In these genera, the raphe is wrapped around the margin of each valve and raised in a special structure called a wing. Cells are usually relatively large.