"...And St. David!"

During the Boxer Uprising (1900), at Tientsin and Peking, the Marine Battalion in the international relief column was Brigaded with the Royal Welch Fusiliers (23d Foot), one of Britain's most renowned Regiments. The resulting fellowship between the two organizations is symbolized each year on St. David's Day (1 March, the Welsh national holiday), when the Commandant of the Marine Corps and the Colonel of the Fusiliers exchange by dispatch the traditional watchword of Wales: "...And St. David."

-- http://marinecorpsbrotherhood.usmchq.com/custom2.html
The Greeting is also exchanged on the Marine Corps Birthday.

btw, the Marines's birthplace, Tun Tavern in Philadelpia, was also the home of the St. Andrew's Society of Philadelphia, first founded in 1747.

Cheers,

Todd