A brass (copper and zinc) or copper Roman coin (dupondius singular, dupondii plural). These are slightly smaller than the sestertii, at 22 to 28mm diameter. On dupondii the Emperor's head is shown wearing a spiky crown (radiate crown). One dupondius was worth two asses.
A Roman dupondius coin, perhaps from Burnham Overy. NWHCM 1938.121.1:A (© NCC)