Retrieved September 25, 2020. ↑ "The Song Dynasty in China | Asia for Educators". afe.easia.columbia.edu. Retrieved 2020-09-25. ↑ "The History of American Currency | U.S. Currency Education Program". www.uscurrency.gov. Retrieved 2020-09-25. ↑ Quinn, Stephen; Roberds, William (November 4, 2008).