This stater of Pergamon has a small Medusa head in the right field. The ΠΕΡ monogram in the left field provides the attribution.
This coin is large, 27mm, and the Medusa head is about 5mm across. I recall F.S. Kleiner & S.P. Noe (The Early Cistophoric Coinage (1977)) date tetradrachms with this symbol to 135-128 BC. I suspect the Medusa head symbol is very scarce. There are none on CoinArchives and this is the only one I was able to spot on eBay. This specimen is reputedly from a 1995 Empire Coins list but I haven't been able to verify.
I don't know if anyone has speculated on the meanings of the symbols on these tetradrachms. I don't have my own copy of Kleiner-Noe but they did not speculate on the Medusa head.
I suspect the Medusa head symbol is related to the head on the very rare Medusa tetradrachms usually attributed to Pergamon.
This coin, which is not in my collection, is the largest silver coin made by the Greeks depicting Medusa (by size; the weight is no greater than the archaic Athenian Wappenmuenzen).
Those tetradrachms, of which only three are known (in Copenhagen, Paris, and the BM), have the inscription ΑΘΗΝΑΣ / ΝΙΚΗΦΟΡΟΥ. Although only three are known mystery of the attribution and the magnificence of the engraving has inspired several articles on them and much creative thinking.
Antonia S. Faita (“The Medusa-Athena Nikephoros Coin from Pergamon”, Athena in the Classical World (2001) argues the balls on Athena, which previous writers believed were female breasts, actually represent the cities of an economic union. She writes “Athena Nikephoros ... wears an apron with seven balls, the largest one in the centre. Her left hand holds a branch with seven leafy shoots. The balls could not possibly have represented breasts, symbols of a fertility cult, as they lack the most important element of fertility, the nipples. The absence of these spherical globes from representations of Athena Nikephoros on coins dated to the Roman period clearly indicates that the globes were not depicted on the goddess’s cult statue. The globes on Athena’s pinafore are placed on an apron tied around her waist far to low to be considered breasts. The equal number of balls and shoots may well mean they alluded to the same idea.” ... “The central ball on the goddess’s apron, being the largest, represented the capital Pergamon, the seat of Attalid power; the other six, the remaining cities issuing cistophoroi.”
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