Commentary Purg VIII 79-81

Nino's heraldic language suggests that the device of the Milanese Visconti's coat of arms, the viper, will not decorate her tomb as well as would have his family's device, the rooster, had she remained a widow and eventually died in Pisa.  According to Iannucci (Iann.1978.1), pp. 4-6, Nino's family drama is resolved in biblical terms, with his wife, Beatrice, associated with the serpent of Genesis, and his daughter, Giovanna, with Mary.

      Whether with a purpose or not, Dante plays off the situation that pertained in V.N.XXIV.3, when Guido Cavalcanti's Giovanna preceded Dante's Beatrice as John the Baptist preceded Christ.  Here the innocent Giovanna is a foil to the unnamed but vicious Beatrice d'Este, her own mother.  In the language of King Lear turned inside out, 'bad wombs have borne good daughters.'