Final answer:
Eridanus is not a river in the Greco-Roman underworld; this mythological realm includes rivers like Styx, Acheron, and Cocytus.
Step-by-step explanation:
The river that is NOT a river in the Greco-Roman underworld is Eridanus. In ancient Greek and Roman mythology, the underworld is depicted as a realm surrounded by a series of mythical rivers. These rivers include the Styx, the Acheron, Cocytus, and Lethe, among others. The Styx is often considered the primary river that the souls of the deceased must cross, typically ferried over by Charon, the ferryman of the dead. The river Eridanus does not figure in this mythological schema of the underworld and is therefore not one of the underworld rivers according to Greco-Roman tradition.