The Roman god Janus appears on some of the earliest coins of the Roman Republic, appearing about 240 B.C. during the Pyrrhic War. Theaes grave, some of the earliest cast Roman coins, often featured the two-headed visage of Janus. Theaes grave were the first coins minted in Rome before the familiar denarius which were to become the most used and well-known coins of the Roman republic. Theaes grave were the successors to the aes crude, irregularly shaped lumps of metal that were used as a medium of monetary exchange. The aes crude were unwieldy and inconvenient because each had to be weighed every time they were exchanged. The introduction of theaes grave introduced a degree of standardization.
The earliestaes grave appeared in southeastern Italy around 289 B.C., and weighed about 320 grams (3/4ths of a pound). They were round, made of bronze, and featured the two-headed Janus on one side, and a ship's prow on the reverse. The as became the basic unit of exchange for the next 75 years or so.
Interestingly, the denarius of the Roman Republic featured some of the most diverse themes and designs found on coinage in nearly any other period in history. The minting system clearly defined weight and purity of metal, but left the moneymakers a great deal of leeway in choosing specific designs to feature on their coins. Thus, rare silver denarius coins depicting the head of Janus exist. Much more common, though, is the head of Roma, and depictions of famous classical scenes.
Janus - God of gods
One of the more interesting of the Roman gods is the god Janus, usually depicted with two heads joined at the back and facing in opposite directions. He was the god of opposites - of exits and entrances, beginnings and endings, doors, doorways, gates and crossroads. As such, he was the natural patron on the month of January, the gateway to a new year. He represented transitions and progress, the changing of one state to another, and was often invoked at the beginning of planting and the end of the harvest. He is also uniquely Roman. Unlike most other gods of the Roman pantheon, he has no ready counterpart in the Greek pantheon, though he may have originated in Thessaly, in Greece.
Janus was considered to be a benevolent king of Rome's Golden Age. His reign introduced cultivation, the law and money. Thus, he also represents the transition from barbaric to civilized culture, from rural to urban. Janus earned the status of protector of Rome when Romulus, one of Rome's mythological founders, kidnapped the Sabine virgins. According to the story, the Sabines attempted to enter the city and revenge themselves for the kidnapping by climbing one of the Seven Hills. Janus caused a hot spring to erupt from the ground before them, and they fled. Since then, Roman tradition called for the doors to the Temples of Janus to be open during wartime, so that he could more easily step forth to protect his city.
Those doors were referred to as the Gates of War, and they have quite a tradition throughout Roman history. The Gates of War are the double doors of the Temple of Janus on the street named Argiletum, on the Roman Forum road, through which the Roman legions passed on their way to war. The Temple has two doors - one facing east and one west, toward the rising and setting sun. The tradition that those gates would always stand open during times of war was mentioned by Plutarch, Virgil, Augustus, Eutropian and numerous other ancient writers, though they all attribute slight differences to the tradition. Virgil, for instance, claims that in times of peace, the doors are closed to keep war imprisoned, lest her lust for blood overtake the land again. Ovid and Horace, writing at about the same time, claim that the gates are closed to hold Peace within the city instead.
In any case, the importance of Janus in everyday Roman life during the years of the Republic and earlier is indisputable. Every meal was begun with a request for his blessing, and public ceremonies began with a libation drunk to the god of beginnings. He was the 'god of gods', appearing first on the most ancient lists of Roman gods - so it's only fitting that the head of Janus appears on some of the oldest coins of the Roman Republic.