Difflugia labiosa Wailes, 1919, incertae sedis
Syn: Difflugia amphora Penard, 1901
Diagnosis: test ovoid or pyriform, circular in cross-section, colorless to brown, usually composed of small flat pieces of quartz; fundus rounded or slightly tapering to a rounded, usually somewhat excentric apex; pseudostome relatively large, circular or lobed, with or lacking a collar; zoochlorellae often present.
Dimensions: Penard (1902) 150-275 µm, Wailes (1919) 265 µm; Gomaa et al. (2017) 177-208 µm; my measurements: 175-283 µm.
Ecology: in sediment of ditches and lakes. Among all Difflugia shells which I find, shells of Difflugia labiosa are rare. I found this species in sediment of ditches near the city of Montfoort and in the nature reserve Naardermeer in the Netherlands.
Remarks: As pointed out by Gomaa et al. (2017), this species doesn't belong to genus Difflugia but is closely related to Arcella.
Hoogenraad & De Groot (1940) mention a length of 85-160 µm, but both these measurements and their drawings suggest that they have never seen this species, but probably Difflugia amphoralis Cash, 1909.
Ogden considers D. amphora (=D. labiosa) to be a synonym of D. amphoralis, but Cash writes that his D. amphoralis is distinct from D. amphora Penard: "In the latter (not observed in Britain) the mouth has a peculiar chitinous rim with a wavy and sharply cut outline. (...) Penard's D. amphora, which is not a variety of D. urceolata, but a distinct and well-marked species."
An important difference between both species is the size: D. labiosa is much larger (> 150 µm), while D. amphoralis is about 115 µm long.
Wailes (1919) notices: "This species as described by Penard (1902) has a well-characterised test and appears to be quite distinct from D. amphora Leidy (1874), which was afterwards recorded as D. urceolala var. amphora Leidy (1879) and includes forms which belong to D. amphoralis Hopkinson (1908); from all these D. labiosa is distinguished by the recess around the base of the neck of the test, and the wavy collar following the undulations of the polygonal aperture.
In Great Britain it is rare, being only recorded from a single locality, and of the following dimensions: length 265 μm; greatest diameter 160 μm; aperture 65 μm. (Collected by H. E. Forrest from N.W. Anglesey.)"