Can you post a scan? From the description they sound like mother of pearl Loo Chips. Made in Asia as early as the 18th Century (tho most made in the 19th Century] for the English carriage trade.

When I was last in London, I used to have to fight the little old ladies who would grab up the serrated ones and use them to wind their thread around.

You can see scans of them occasionally on eBay and if you have access to Seymour's Antique Gambling Chips you will find pictures on page 12 with pictures of bone chips, including fish, on pages 377 et seq.