The twelfth part of the As. It bears on the obverse the head of Mars, and on the reverse the prow of a galley. On each side is one boss to indicate its weight of one ounce.
A smaller coin, the Semi-Uncia, was of one half the weight and value. See Aes Grave.
Source: Frey's Dictionary (American Journal of Numismatics, Vol. 50, 1916)