A typical series of the Roman heavy bronze money is exhibited (No. =38=; fig. 14) The system is based on the pound of twelve ounces, and the denominations of the various pieces are distinguished by the heads or obverse types, and by the marks of value which they bear. The series consists of the _As_, or pound (+I+), the half, _Semis_ (+S+), the third, _Triens_, of four ounces (····), the quarter, _Quadrans_, of three ounces (···), the sixth, _Sextans_, of two ounces (··), and the _Uncia_, or ounce, the lower unit (·) (_cf._ p. 160). Each of these is further differentiated by the obverse head. The _as_ has the double head of Janus, the god of beginnings, whose coin opened the series of money, as his month begins the year. The _semis_ has the head of Jupiter, wearing a laurel wreath; the _triens_, Minerva armed; the _quadrans_, Hercules in the lion-skin; the _sextans_, Mercury, the messenger, with wings in his cap; and the _uncia_, a head of Bellona, the goddess of battle. All the reverses have a common type, the prow of a ship. This device may mark the date of the introduction of the Roman coinage, which coincided with Rome's first essays on the sea, in the middle of the fourth century before Christ. It remained as the reverse type of the bronze money all through the Republic, and even in later times, when a coin was tossed, the cry was "heads" or "ships."[11] The heavy bronze coinage of the city of Rome was only one among many similar currencies of the central Italian states. As the Romans conquered the neighbouring territories, where there existed local weight-systems, which, in the interests of commerce, it was well to preserve, instead of imposing their own money, they inaugurated subordinate issues at the dependent mints. On this principle it was natural that when the march of Roman conquest came upon the peoples of South Italy, where a silver currency had been long ago introduced by the Greek colonists, a local issue for those parts was instituted as a subsidiary coinage. To this class of Roman money belongs the silver stater or didrachm with Campanian types (the head of Mars and the bust of a horse) which was struck by the Romans--as the legend +ROMANO+(_rum_) shews--in Capua for the use of the Campanian district (No. =39=; fig. 15_a_). With the extension of power and territory the old bronze pieces were inadequate, and in the year 268 B.C. a silver coinage was begun at Rome itself. At the same time the Campanian mint was closed, and the heavy bronze coins, being subordinated to the silver unit, were issued as token-money in a reduced and more convenient size.