A really fine image, Ecki!
What was the valve like? I don`t see the lid, here, though the sides are visible.
A Thuricola, probably.
A possible candidate would be Thuricola kellicottiana, as described by Penard under the name Cothurnia kellicottiana. Kahl moved it to Thuricola. Originally, Stokes had placed it in the new genus Thuricolopsis...though the creature he illustrated (Stokes, 1887) strikes me as quite different in appearance).
When your Kahl comes back from the doctor, have a look at s.785, image 5 (after Penard, not Stokes).
A handy online key:
http://www.peritrich.de/kahl/sessilia/v ... uricol.htm