Senckenberg Naturmuseum, Frankfurt am Main, Germany.
This is a gorgeous fossil shell photographed at the Senckenberg Naturmuseum .. how old is this? What species is this? I don't know .. but I will return and collect more detailed information to share with you (this first visit was a quick pass through the museum to see what they have on display).
If you get a chance go to Solenhofen in Bayern, which is where a lot of the excellent fossils come from. In particular archioptrix fossils come from there. (Being as you are big with birds going to see where their ancestors lived might be interesting) There are museums in both Solenhofen and Eichstadt.
lyle: of COURSE i shall go to both those museums! i would love to go behind the scenes (or behind the glass, as it were) for photography purposes, so am practicing so i will be good by the time i make it there .. hopefully, i will visit soon.
It looks like the inner mold of a large planispiral gastropod. Reminds me of the Ordovician maclurites, but extra-whorly.