See Tacitus, Annal. xi. 11; and Dion Cassius, 43. c. 23, and 49. c. 43. The lowest cast was four points, and it was called Canis. This is one explanation. But the Venus is also explained to be the throw which resulted in all the dice turning up with different faces. See the notes in Burmann's edition of Suetonius, Octav. Augustus, c. 71.