Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Sȳros, i, f., = Σῦρος, one of the Cyclades, between Delos and Paros, now Syra, Mel. 2, 7; Plin. 14, 12, 22, § 67; Ov. M. 13, 175.
Hence, Sȳrĭus, a, um, adj., of or belonging to Syros, Plin. 33, 12, 56, § 158.