This is a Microchlamyiidae (Ogden, 1985) species. Microchlamyiidae is parallel to Arcellidae (Ehrenberg 1843) and has flexible tests. 
63x, DIC
63x
63x, nucleus and CV
The genus Microchlamys (aka Pseudochlamys) has two species: the type species Microchlamys patella (Claparéde & Lachmann, 1859) and Microchlamys sylvatica (Golemansky, Skarlato & Todorov, 1987). M. patella has a diameter of approximately 40 µm and M. sylvatica 140 µm.
The test is areolated (0.45-0.60 µm) and made of Chitin. Young specimen are almost colorless, older Microchlamys are brown (like Arcella). The plasma body is surrounded by a sac with a ventral pseudostome. Image 2 shows both the test as well as the sac.
Microchlamys has a single, central nucleus (image 3).
There is a similar family Spumochlamys (Kudryavtsev & Hausmann, 2007) where the sac is only ventral and the dorsal side of the amoeba is directly attached to the test. The type species was found in a salty lake in Russia. I think my amoeba is Microchlamys patella, but it could be as well Spumochlamys iliensis (Kudryavtsev & Hausmann, 2007). It remains unclear whether the absence of the dorsal part of the sac can be recognized with a light microscope.
Best regards,
Ecki