Passage 8.25.2
τοῦ δὲ ἱεροῦ τούτου σταδίους τεσσαράκοντα μάλιστα ἀφέστηκεν ἡ πόλις· τεθῆναι δὲ αὐτῇ ὄνομα ἀπὸ νύμφης λέγουσι Θελπούσης, ταύτην δὲ θυγατέρα εἶναι Λάδωνος. τῷ δὲ Λάδωνι ἄρχεται μὲν τὸ ὕδωρ ἐν πηγαῖς τῆς Κλειτορίας, καθὰ ὁ λόγος ἐδήλωσεν ἤδη μοι· ῥεῖ δὲ πρῶτον μὲν παρὰ Λευκάσιον χωρίον καὶ Μεσόβοα καὶ διὰ τῶν Νάσων ἐπί τε Ὄρυγα τὸν καὶ Ἁλοῦντα ὀνομαζόμενον, ἐξ Ἁλοῦντος δὲ ἐπὶ Θαλιάδας τε καὶ ἐπὶ Δήμητρος ἱερὸν κάτεισιν Ἐλευσινίας.
The city stands around forty stades distant from this sanctuary. They say it was named after the nymph Thelpusa, who is said to have been a daughter of Ladon. The water of the Ladon begins in springs at Kleitoria, as I have already described. It flows first near the place Leukasion and Mesoboa, then through the Nasoi, on toward Oryx, also called Halous; and from Halous it descends toward Thaliades and the sanctuary of Demeter Eleusinia.