Below the original description of Pyxidicula invisitata by Awerinzew (1906), translated from Russian by Yana Eglit:

 

The test in its form is reminiscent of the previous species (P. cymbalum): its height is likewise smaller than the diameter, and the oral aperture occupies the entire ventral surface of the test, and thus the oral diameter matches that of the test.
Under higher magnifications (Zeiss objective E, occular 4) one can resolve a cellularised/mesh-like structure of the test, by combining pictures of longitudinal (table V fig 63) and transverse optical sections.
P. invisitata differs from P. cymbalum in its diminutive size, and, most importantly, that the little lip surrounding the shell attaches to its outside surface in the former -- a bit removed upwards from the edge of the test. As a result, the test of P. invisitata in its transverse section resembles an arc with bifurcating ends (table V fig 61, 62). The nucleus is vesicular with one nucleolar body (table V fig 64).

Diameter of P. invisita test = 0.040-0.050 mm
Habitat: swampy creeks
Russia (Archangel'sk region)

Pyxidicula invisitata

 

Fig 61. Overall semi-schematic view of the P. invisitata.
62. Optical transverse section of the test.
63. Part of the test with its surface.
64. The nucleus.