The unit of value in the Roman and old Italian coinages, was made of copper, or of the mixed metal called Aes. It was originally of the weight of a pound of twelve ounces whence it was called as libralis and aes grave. The oldest form of the as is that which bears the figure of an animal. The next and most common form is that which has the two-faced head of Janus on one side, and the prow of a ship on the other (whence the expression sed by Roman boys in tossing up) - Smith, 1873