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 his specimen, published by Deflandre (1929) has in my opinion nothing to do with C. marsupiformis.
This species resembles species of the genus Collaripyxidia.