The Greeks used only a few copper coins, but the Romans had a large variety of copper money. The Roman As probably signified originally 1 lb weight of uncoined copper. It was divided into twelve ounces, the first six of which were represented by copper coins. The earliest Roman currency comprised copper or bronze cast bricks, upon which was stamped the figure of an ox. This money was weighed - splendid specimens of their early bronze balances still survive - and it may be that copper coins resulted from the necessity of having pieces of uniform size, thickness and weight in order to provide accurate small change. The earliest Roman copper coins were not struck, but were cast in stone moulds. Subsequently the impression, a two-headed Janus on one side and the prow of a ship on the other, was struck on the plain discs, which were placed on an anvil. In Imperial times the head of the reigning Caesar generally appeared on the face of Roman coins.