I know this photo isn't much to go by but I didn't have a better camera and no tools to extract it from the large rock so I could bring it home. I just hope I can still find it.
This is about 10 cm long, found in Spring Creek, Cooke County, Texas. These are marine, limestone deposits with abundant ammonites. It may not be bone but I can't think what else it might be. That could be a scaphopod below it but I haven't seen them here before.