This seems to be the earliest coin (c.53BC) in which the symbols of Hellenistic kingship, the diadem and the scepter, are used in such a way as to suggest their rejection in favor of the traditional symbols of Roman power in this case the curule chair. For this coin, the context is the threat of Pompey assuming sole control of the Roman state.
We see a similar iconographic strategy on a coin of Brutus after the murder of Julius Caesar:
The question in my mind is should a similar interpretation also apply to this type:
The auction houses invariably photograph this specimen with the orientation shown above, but Crawford had his plates printed at the 90 degree rotation of the reverse:
Crawford is silent on the symbolism of diadem saying only: “Part of one issue of Cassius records his capture of Rhodes after a battle at Myndus, opposite the island of Cos; the rose of Rhodes and crab of Cos both figure, together with an aplustre as a symbol of victory” (p. 741). I must say, the aplustre doesn’t seem very victorious to me as it is clutched in the claws of the crab. Or perhaps its just Cos offering the naval victory to Cassius…
I also think I prefer symbolically the crab and aplustre read as over and above the more diminished Rhodian rose and the diadem, just as the curule chair symbolically sits over the diadem and sceptre in the first type above.
The Cassius coin is often connected to this quote from Plutarch’s Life of Brutus:
“Cassius, having taken Rhodes, behaved himself there with no clemency; though at his first entry, when some had called him lord and king, he answered that he was neither king nor lord, but the destroyer and punisher of a king and lord.”
I’m not sure this is specifically the allusion the die engraver was aiming at but it is certainly a reflection of the same rhetorical impulse.
1/17/16: on Crabs on coins: