Difflugia microclaviformis (Kourov, 1925)
Diagnosis: Test pyriform, usually with a distinct aboral protuberance; test circular in cross-section, colorless to brown, usually composed of small to medium pieces of quartz; smooth surface and a well defined outline; protuberance often eccentrically placed; neck sometimes slightly bent; pseudostome circular, surrounded by small particles; zoochlorellae observed; nucleus spherical, with ring-shaped nucleolar material scattered through the nucleus.
Dimensions: Kourov 185 µm; Ogden 202-206 µm (n=2); my measurements: 151-186 µm (n>30).
Ecology: In sediment of ditches.
Remarks: This species might be the same as Difflugia acuminata var. curvata Cash and Hopkinson, 1909
Shells of Difflugia microclaviformis seem to be rare. I found this species only in the sediment of two ditches in the nature reserve Naardermeer (north and south) in the Netherlands. Specimens at both locations were abundant and alive. Between the typical specimens, with protuberances, also specimens with a smooth fundus were present (see the photo below).