Silver siglos, 17mm, 5.35gm, issued c. 340-320 BC.
Obv: KAΛX, Bull standing to left on corn ear.
Rev: Quadripartite incuse square of mill sail pattern.
References: Sear 3738.
The Bosporus (between the cities of Byzantion and Kalchedon) 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.