Silver siglos, 18mm, 5.30gm, issued c. 340-320 BC.
Obv: Cow standing on dolphin, both to left, ΠY monogram above.
Rev: Quadripartite incuse square of mill sail pattern.
The Bosporus (between the cities of Byzantion and Chalcedon) meant "ox passage", where the Greek myths believe Io, disguised as a bull crossed between Asia and Europe. Both city states use the same iconography.