Cf. Exhibition catalogue, Égypte Romaine, Marseille, 1997, pp. 260-261, no. 293 for a bronze janiform herm of Zeus Ammon.
Ammon was an Egyptian oracle god whose main sanctuary was situated in the Siwa oasis. The Greeks were the first to call the god Zeus Ammon and his cult spread throughout the Greek world, made famous in the 4th Century B.C. when Alexander the Great visited the oracle and was greeted as Ammon's son. In the Roman period the god was still popular, Augustus using images of the god in the Forum in Rome and his cult spread as far as the river Rhine.