The inscription translates as: Arethusa changed into a river.
In his Metamorphoses, Book V: 572-641, Ovid relates the tale of Arethusa who, returning from hunting with Diana, stopped to refresh herself in the River Alpheus. The enamoured god of the river pursued the nymph relentlessly across the hilly landscape. Just as he was about to embrace the desperate and exhausted Arethusa, she asked Diana for protection. The goddess enveloped her in a cloud and changed her into an underground stream, so that her waters would never mingle with those of Alpheus. The nymph is shown at the right of the dish bathing in the River Alpheus and at the left hiding beneath a veil (cloud) just before she is transformed.