Planothidium have a raphe slit on only one of the two valves. This valve is used to attach the cell to a surface. Planothidium lanceolatum is a freshwater species.
Planothidium, species undetermined, found in diatomite deposit at Three Lakes, core TL5, at 3.6 to 3.7 m depth.
Planothidium lanceolatum found in core TL1 at 5.95 m depth.
See Planothidium on Algaebase or
Diatoms.org or on the
Natural History Museum of Wales website.