Chief Ranger Bob Grant says the theft from a locked case at the Salt Pond Visitors Center in Eastham was discovered Monday.
He tells WXTK-FM that one piece is a 22-inch long walrus tusk with the engraving “Ship. Wm. Thompson.” The other is a 3 1/2-inch walrus head carved from a piece of walrus tusk.
He did not have an exact dollar value of the pieces, which date to the 19th century and have been on public display since 1989.
National Parks Services investigators are getting help from local law enforcement looking for the pieces.
The museum will remain open during the investigation.