Here’s the information from the US Navy’s NAVPERS 15.790 of 1948 (Navy’s Awards Manual):
(3) A bronze star, three-sixteenths of an inch in diameter, will be placed on the service ribbon in lieu of clasp authorized . When any person has been commended as the result of the recommendation of the board of awards by the Secretary of the Navy for performance of duty during World War I not justifying the award of a Medal of Honor, a Distinguished Service Medal, or a Navy Cross, he shall wear a silver star for each such citation.
(4) A bronze Maltese cross, three-sixteenths of an inch in diameter, will be placed on the service ribbon for those officers and men of the Marine Corps and Medical Corps, United States Navy, who were attached to the American Expeditionary Forces in France any time between 6 April 1917, and 11 November 1918, and who are not entitled to any battle clasp provided for by General Order No. 83, War Department, 30 June 1919.
Hope that helps you out.
This reply was modified 3 years, 3 months ago by megan. Reason: Removed extraneous formatting codes