Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Tīthōnus or -nos, i, m., = Τιθωνός, son of Laomedon, consort of Aurora, and father of Memnon; endowed with immortality, and changed at last, after reaching a decrepit old age, into a cicada, Cic. Sen. 1, 3; Verg. A. 4, 585; Ov. F. 6, 473; Hor. C. 2, 16, 30; 1, 28, 8: Tithoni conjux, Ov. H. 17 (18), 111.
Hence,