Bosphorum:
Mithridates had been driven to
Byzantium, a city located
here on the
Bosphorus.
annis quadraginta:
The First Mithridatic War lasted from 88-84 BCE, the Second from 83-81 BCE, and the Third from 75-63 BCE.
Though annis appears as an ablative, treat it as an accusative agreeing with
quadraginta to show time duration.