Piece of Eight
This is a 1740 Spanish Dollar - worth 8 Spanish Reales - hence the name. This particular coin was minted in Mexico City and was on its way aboard the Dutch East Indiaman 'Hollandia' as part of a cargo of silver for trading in the Far East.
She sank off the Isles of Scilly on her maiden voyage from Amsterdam to Batavia in July 1743 with the loss of all hands.
The wreck was discovered by divers in September 1971 and a large quantity of coins were recovered, of which this is one.