Difflugia microclaviformis (Kourov, 1925) Ogden, 1983
Basionym: Difflugia oblonga var. microclaviformis Kourov, 1925

Diagnosis: Shell 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 (1925): Shell length 139-160 µm. Ogden (1983) 202-206 µm (n=2). Ogden and Zivkovic (1983) 141-163 µm (n=2). My measurements: 137-186 µm (n=32, Netherlands); 178-228 µm (n=5, Spain).

Habitat: Freshwater. In sediment of ditches and rivers. Europe.

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 sediment of two ditches in 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). I found specimens in sediment of River Águeda, near Ciudad Rodrigo, Spain, 2025.

The variety described by Cash is probably identical to D. microclaviformis. Here is the description from Cash (1909):

Var. curvata 
Shell pyriform, curved, with a short cylindrical neck, and a blunt mammillary protuberance on the crown; its two sides (as seen in one aspect) unequally convex; the pronounced curvature of the test produced by the outer surface from the crown downwards, for about three-fourths of the total length, being broadly convex whilst the opposite side is convex for only one half the total length, or less, and then carried downwards, concavely in outline, so as to form the narrow neck. (In another aspect of the test -that is, viewed with the broader convex surface towards the eye- the sides appear straight and approach each other gradually till they reach the abruptly-truncated mouth. In this aspect, moreover, the crown has a tendency towards the acuminate form, though the shell is not in the least compressed, and the coronal protuberance is slightly deflected from the longitudinal axis.) External surface overlaid with muddy particles, and opaque; consequently the protoplasmic contents of the test are invisible. Pseudopodia normal, few in number.
Dimensions: Length 240 um; breadth 110 um.
In pond at Chipperfield, Herts, 1904 (J. Earland). This variety seems to be peculiar to the Chipperfield pond. A considerable number of individuals and some empty tests were examined and all exhibited the same remarkable curvature. The coarser Difflugia exist in this pond in great abundance.* The quality of the water, the nature of the soil, or some other cause unknown, appears to favour their growth and development. The variety under notice occurs in association with D. oblonga Ehrenb., D. crassa sp. nov., D. urceolata Carter, Lesquereusia modesta Rhumbler, and other species.
• [A small pond overgrown with rushes, on the south edge and near the cast end of Chipperfield Common.]