Greek coin of King Prusias II
Size: 16.5mm Weight: 3.52 grams
Obverse: Head of king Prusias to right, with a winged diadem 
Reverse: BAΣIΛEΩΣ – ΠPOYΣIOY, Heracles standing left, holding club set on ground in his right hand and lion skin in his left, monogram in right field.

Waddington, Recueil Géneral 25 (monogram 3); HGC 7, 634. 

The Kingdom of Bithynia (Ancient Greek: Βιθυνία) was a Hellenistic kingdom centered in the historical region of Bithynia, which seems to have been established in the fourth century BC. In the midst of the Wars of the Diadochi, Zipoites assumed the title of king (basileus) in 297 BC.[1][2] His son and successor, Nicomedes I, founded Nicomedia, which soon rose to great prosperity. During his long reign (c. 278 – c. 255 BC), as well as those of his successors, Prusias I (r. c. 228 – 182 BC), Prusias II (r. c. 182 – 149 BC) and Nicomedes II (r. c. 149 – 127 BC), the Kingdom of Bithynia prospered and had a considerable standing and influence among the minor monarchies of Anatolia. But the last king, Nicomedes IV, was unable to maintain himself in power against Mithridates VI of Pontus. After being restored to his throne by the Roman Senate, he bequeathed his kingdom through his will to the Roman republic in 74 BC and it became the province of Bithynia et Pontus in 63 BC.[3][2]

The coinage of these kings show their regal portraits, which tend to be engraved in an extremely accomplished Hellenistic style.