Have found several of these over time in late 1800s-early 1900s sites, the last one today, and have always called them buttons. However I am not entirely sure? (penny for reference-and for your thoughts) Thanks for your help.
Thank you jeweleryguy and Bruce R for your replies. It appears that you are both right. They are Victorian Era white milk glass and brass button covers and/or picture nail covers. Apparently they were used both ways. Thanks to both of you again.