2. SER. CORNELIUS (M. F. L. N.) COSSUS probably brother of the preceding, was consul in B. C. 428 with T. Quinctius Pennus Cincinnatus II., and two years afterwards, B. C. 426, one of the four consular tribunes, when he was entrusted with the care of the city, while his three colleagues had the conduct of the war against Veii. But the latter having met with a repulse, Cossus nominated Mam. Aemilius Mamercinus dictator, who in his turn appointed Cossus master of the horse.

It was this Cossus who killed Lar Tolumnius, the king of the Veii, in single combat, and dedicated his spoils in the temple of Jupiter Feretrius--the second of the three instances in which the spolia opima were won. But the year in which Tolumnius was slain, was a subject of dispute even in antiquity. Livy following, as he says, all his authorities, places it in B. C. 437, nine years before the consulship of Cossus, when he was military tribune in the army of Main. Aemilius Mamercinus, who is said to have been dictator in that year likewise. At the same time the historian brings forward several reasons why this was improbable, and mentions in particular that Augustus had discovered a linen breastplate in the temple of Jupiter Feretrius, on which it was stated that the consul Cossus had won these spoils. But as the year of Cossus' consulship was, according to the annalists, one of pestilence and dearth without any military operations, it is probable that Tolumnius was slain by Cossus in the year of his consular tribunate, when he was master of the horse, especially since it is expressly placed in that year by some writers. (V. Max. 3.2.4; Auir. Vict. de Vir. Ill. 25.) In dedicating the spoils, Cossus would have added the title of consul, either on account of his having tilled that dignity or in consideration of his holding at the time the consular tribunate. (Liv. 4.19, 20, 30_32; Plut. Romul. 16, Marcell. 8; Niebuhr, ii. p. 458, &c.; Propert. 4.10. 23, &c., who gives quite a different account.)