Lewis & Short

Zăcynthus or -ŏs, i, f., = Ζάκυνθος, an island in the Ionian Sea, now Zante or Zacyntho, Mel. 2, 7, 10; Plin. 4, 12, 19, § 54; Liv. 21, 7; 26, 24; Verg. A. 3, 270; Plaut. Merc. 3, 4, 62; Ov. H. 1, 87 al.
Hence, Zăcynthĭus, a, um, adj., of or belonging to Zacynthus, Zacynthian: Calchas, Plaut. Merc. 5, 2, 104: bitumen, Plin. 35, 15, 51, § 178.