UCMP MYSTERY FOSSIL #46 is . . .
the gastropod Bellerophon hilli

Yes, Bellerophon hilli may look like a small nautiloid, but it is a gastropod (no septa like a nautiloid). Bellerophontid gastropods appeared in the late Cambrian and reached their highest diversity during the Ordovician and Silurian. This silicified specimen was found in Permian limestones in Shasta County, California. The UCMP website has a gap in its information on gastropods in general but you can read more about bellerophontids here.