Commentators on this passage have drawn attention to the unusual genitive in the phrase ferox scelerum, ‘fierce in his crimes’: ‘this adj. seems here alone to take an objective genitive’, says Furneaux, while Martin and Woodman state that ‘the dependent genitive of an external attribute, evidently on the analogy of its use with personal characteristics (e.g. Ovid, Met. 8.613 mentis), seems unparalleled and is perhaps intended to suggest that Sejanus' criminality was innate’. Most commentators add a reference to Sallust's description of Jugurtha as sceleribus suis ferox (Jug. 14.21), but that passage is no help as a parallel for the construction, since it gives the ablative usual after ferox in Tacitus and other writers to describe the reason for ferocity (cf. Agr. 27.1 fama ferox, Hist .1.51.1 ferox praeda, Ann. 1.3.4 robore corporis stolide ferocem, TLL 6.567.76ff.).