"Medal". En Nicholson, William. American edition of the British Encyclopedia or Dictionary of Arts and Sciences. Vol. VII. Ed. Mitchell, Ames and White. Philadelphia, 1821: "Servius Tulius introduced the practice of impressing figures on their copper or aes (...) This manner of distinguishing the coin was afterwards changed, and janus on one side, and the prow of a galley on the other, became the marks of the aes".
