On the diminution in size having reached a limit in any species, the maximum size is regained by the formation of an auxospore.

In the figure one valve alone of the auxospore is seen, the opposite valve not being in focus.

The valves of the auxospore are usually more or less arcuate, as in Cymbella, to which the genus is closely allied.

The sagittal plane of the valve of the auxospore is at right angles to the plane of the valve of the mother cell.