Centropyxis marsupiformis (Wallich, 1864) Deflandre, 1929 pars
Diagnosis: Test large, more or less ovoid or elongate elliptical, somewhat compressed, without or with one to six spines or horns at the posterior part; aperture elliptic or sub-circular, perhaps even sometimes circular, located close to the anterior border. In lateral view elliptical, truncated at about 50-75°. Test embedded with mineral particles. Spines hollow and sometimes closed by a small spiny particle.
Dimensions: Leidy: Length incl. spines 180-340 µm, width 120-180 µm; inclination 50-60°. My measurements (2017): incl. spines 203-244 µm long (n=14).
Ecology: sediment of ponds and ditches; North America, the Netherlands (Naardermeer).
Remarks: When comparing the text of Deflandre (1929) with Leidy’s original description (1879), there are some discrepancies.
Deflandre writes in his diagnosis that the apertural plane is truncated at about 45°, but I cannot see how he achieved that result. Leidy (1879) describes this inclination as follows: “…when the animal is erect, as in its ordinary movements, the long axis, corresponding with a line passing from the centre of the mouth to the summit of the shell, is oblique…” When I measure the angle of Leidy’s tests according to his description, the results are c. 50-60°.
Secondly Deflandre writes that the largest shells found by Leidy (“in the text”) are 340 µm long and 140 µm broad, but Leidy names a width of 180 µm.
According to Deflandre (1929) tests with 0-2 spines belong to C. marsupiformis while tests with 3-6 spines belong to his variety obesa. However Leidy found tests with 1-6 spines in the same pond (Absecom pond, N.J.). I also found my tests, with 1-6 spines, in one ditch, even in one small sample. So I cannot see how there is a variety “obesa“. The drawing of this specimen, published by Deflandre (1929) has in my opinion nothing to do with C. marsupiformis.
This species resembles species of the genus Collaripyxidia. It might be identical with Difflugia azerbaijanica.