I couldn't find a suitable match in Albert's book, but in Tice's book, on page 381, button 10 fits yours quite nicely. According to Mr. Tice, your button was made around 1860.
ALL I CAN SAY MCDIG THAT IS A HELLUVA FIND AND I CALL IT THE TURTLE SHELL EXCELSIOR BUTTON , FOR THE DESIGN OF THE DOME LOOKS LIKE A TURTLE SHELL. MAN THAT IS A NEAT ONE YOU GOT THERE.