Ancient term (Plin. HN 33,46; Fest. p. 98 and 347B; Tac. Germ. 5; Liv. 23,15,15; 34,10,4. 7) for the denarius, with a carriage depicted on the reverse carrying a deity (Diana, Hercules, Luna, Victoria i.a.). In Livy (33,23,7. 9; 34,46,12; 36,21,11), a synonym for denarius (argentum bigatum). Current opinion has the first bigatus coins minted from 189/180, and the last around 42 BC.
R. Thomsen, Early Roman Coinage. A Study of the Chronology, 1-3, 1957-61, s.v. Bigatus
RRC2, 613f., 630.