Hi Heribert,
Surely the Mediopyxis works well in stacking. And without

The centric looks very much like Thalassiosira angstii, but I'm used to see it a lot finer areolated in our material. So I'm surprised to see it resolved with your 20x lens.
I'm still having trouble with the Rhizosolenia ID, I generally would call both of them R setigera, but I think we're messing up R. setigera with R. pungens and maybe some other cryptic species.
Best wishes, René