Coins were not placed on the eyes; all literary sources specify the mouth. Callimachus, Hecale fragment 278 in R. Pfeiffer's text Callimachus (Oxford UP, 1949), vol.2, p. 262; now ordered as fragment 99 by A.S.D. Hollis, in his edition, Callimachus: Hecale (Clarendon Press, Oxford 1990), pp. 284f., from the Suidas, English translation online, specifying the mouth, also Etymologicum Graecum ("Danakes"). See also Smith's Dictionary of Greek and Roman Biography and Mythology, entry on "Charon" online for placement in the mouth, though archaeology disproves Smith's statement that every corpse was given a coin; see article on Charon's obol.
Christiane Sourvinou-Inwood, "Reading" Greek Death (Oxford University Press, 1996), p. 359 online and p. 390 online.
