Many Planothidium species, but not all, possess an asymmetrical central area, which bears internally either a rimmed depression or a hood. The difference between these two structures was not recognized before SEM studies, and both of them were termed ‘hufeisenförmige Fleck” in German or “hoof-mark” or “horseshoe-shaped area” in English. The rimmed depression is a structure found only in the genus Planothidium. It is a hyaline structure on one side of the central area of the rapheless valve.
The German term used to describe this structure is also used in English floras.