Hi Science Girl, you've probably received tons of emails from other geologists, but your mystery object is an ammonite fossil, a nautiloid sea creature which lived between the Silurian/Devonian (ca. 400 million years ago) and became extinct with the dinosaurs at the end of the Cretaceous (65 Ma). Definitely one of the more spectacular fossils! Nice photos and content on your site by the way.
Cheers,
Jacques
Geologist
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Having been in the NHM this morning, the 'missing' plaque identifies it as Asteroceras Stellare from the Early Jurassic
If you examine my comment, you'll see I hid the correct answer in a link.
A few other people guessed ammonite as well.