other belief systems that are culturally unrelated. The boatman of the dead himself appears in diverse cultures with no special relation to Greece or to each other. A Sumerian model for Charon has been proposed, and the figure has possible antecedents among the Egyptians; scholars are divided as to whether these influenced the tradition of Charon, but the 1st-century BC historian Diodorus Siculus thought so and mentions the fee. It might go without saying that only when coinage comes into common use is the idea of payment introduced, but coins were placed in graves before the appearance of the Charon myth in literature. Because of the diversity of religious beliefs in the Greco-Roman world, and because the mystery religions that were most concerned with the afterlife and soteriology placed a high value on secrecy and arcane knowledge, no single theology has been reconstructed that would account for Charon's obol. Franz Cumont regarded the numerous examples found in Roman tombs as "evidence of no more than a traditional rite which men performed without attaching a definite meaning to it." The use of a coin for the rite seems to depend not just on the myth of Charon, but also on other religious and mythic traditions associating wealth and the underworld. Death and wealth In cultures that practiced the rite of Charon's obol, the infernal ferryman who requires payment is one of a number of underworld deities associated with wealth. For the Greeks, Pluto (Ploutōn, Πλούτων), the ruler of the dead and the consort of Persephone, became conflated with Plutus (Ploutos, Πλοῦτος), wealth personified; Plato points out the meaningful ambiguity of this etymological play in his dialogue Cratylus. Hermes is a god of boundaries, travel, and liminality, and thus conveys souls across the border that separates the living from the dead, acting as a psychopomp, but he was also a god of exchange, commerce, and profit. The name of his Roman counterpart Mercury was thought in antiquity to share its derivation with the Latin word merces, "goods, merchandise." The numerous chthonic deities among the Romans were also frequently associated with wealth. In his treatise On the Nature of the Gods, Cicero identifies the Roman god Dis Pater with the Greek Pluton, explaining that riches are hidden in and arise from the earth. Dis Pater is sometimes regarded as a chthonic Saturn, ruler of the Golden Age, whose consort Ops was a goddess