The first Roman silver coinage bears the types of the goddess Roma, wearing a winged helmet, and on the reverse the patron deities of trade and commerce, Castor and Pollux, the Heavenly Twins or Dioscuri (No. =40=; fig. 15_b-d_). They are armed with spears and ride on horseback, with their stars above their heads. These types occur on all three denominations of the earliest silver, the _Denarius_ (marked +X+), which was worth 10 _asses_; its half, the _Quinarius_ (+V+); and the _Sestertius_ (+IIS+) of 2-1/2 _asses_, which became the unit in reckoning accounts. The two smallest silver pieces were not always struck; but the _denarius_, with the reduced copper for small denominations, remained in use during the period of the Republic at Rome and long into the Empire. Although both series had a great variety of types, the fabric and general appearance were unaltered. With the change to the Empire, reform in all directions was begun, and the coinage was set on a new basis. Gold was introduced to meet the needs of the metropolis of the world, and two new coins, the _Aureus_ and its half, were struck in this metal. They were modelled on the silver pieces. The standard silver coin was still the _denarius_, and the only change which it experienced was in type. The head of the emperor took the place of those of deities, with a superscription, which was the forerunner of modern coin-legends. It consisted of the name and titles of the emperor, often with the date of striking, arranged in a circle round the edge of the coin. The minting of gold and silver was assumed by the emperor, but the lower denominations were left to the senate, whose authority is expressed on each piece by the letters +S·C+ (_Senatus Consulto_, "by decree of the Senate"). The senatorial series consisted of the _Sestertius_, the equivalent of the smallest silver coin, now valued at 4 _asses_ instead of the original 2-1/2; the _Dupondius_, of 2 _asses_; the _As_, and fractions of the _as_, _Semis_ and _Quadrans_, which are of less frequent occurrence. These coins sometimes differed as to the metal used, the _as_ and _semis_ being of copper, and the _dupondius_ and _sestertius_ of brass; or in the style of the emperor's head; or, as in the case of the coins exhibited, the _as_ is marked +I+ and the _dupondius_ +II+ (fig. 15_h_ and _i_). Usually, however, the two pieces are confused, and are loosely termed by collectors "second brass," the sesterce being "first brass," and all denominations lower than the _as_ "third brass." The reverse types were very numerous, and, with the exception of the mark +S·C+ on the senatorial issues, none of them was peculiar to any denomination. The series which is selected here to illustrate the Imperial coinage is of the reign of Nero (54-68 A.D.); all the pieces, therefore, bear the image and superscription of that Caesar, and their reverses have complimentary references to the emperor and his family, or topical allusions to current events (No.