Parthax wrote Italica. Herodian quotes book 2: Heracles came to Poseidonia, where a great river is called Is. The Etymologicum Magnum adds that the same stream was later Cyllistanos, because Heracles killed a huge snake there and it rolled into the water. Eustathius on Iliad 734.48 records that Parthax called Caulonia Aulonia.
Karl Müller printed the notices in Fragmenta Historicorum Graecorum 4 (Paris, 1841–1870), pp. 466–467 (TLG 1568.002). The Canon dates him after the second century CE (PARTHAX Hist., a. A.D. 2).
The slot is that Italic geography: a river at Poseidonia, a Heracles aetiology, and a south-Italian toponym.
Also published as PDF.