This Roman coin is a semuncia, a half uncia; a small denomination issued only sporadically and in small numbers during the time of the Roman Republic.
At the time of their introduction around 270 BC, semunciae were cast and weighed some 12 grams. However, due to the constant wars that Rome fought in the course of its expansion throughout Italy, the price of bronze increased. This made it possible to lower the weight of the bronze coins without diminishing their value. This semuncia from the time of the Second Punic War (218-201 BC) weighs a mere 3.8 grams. It shows the head of the god Mercury with his winged helmet and a prora, the bow of a ship.