Difflugia biconcava Ertl, 1965
Diagnosis: Shell ovoid or pyriform, with an aboral rounded cone or spine and laterally compressed; shell in lateral view with typical biconcave shape; aperture circular; shell composed of a mixture of small to large pieces of mainly flattish quartz, arranged to give a relatively smooth surface. Nucleus spherical with large central nucleolus which contains a number of lacunae.
Dimensions: Ertl 100-143 µm long, 66-96 µm broad, aperture 29-32 µm. My measurements: Naardermeer 149-190 µm (n=8); nucleus 31-39 µm; Montfoort 78-104 µm. This is an extremely variable species.
Ecology: Ertl found the type material in samples from rice fields in Slovakia. I found this species in sediments of a ditch near the Naardermeer, the Netherlands.
Remarks: D. balcanica Ogden and Zivkovich, 1983 is a very similar, but much smaller species. Ogden gives as length 111-114 µm and as breadth 79-82 µm (n=2).