Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† hēmĭtŏnĭum, ĭi, n., = ἡμιτόνιον, a half-tone, semitone: intervalla tonorum et hemitoniorum, Vitr. 5, 4, 3; Mart. Cap. 9, § 930; 963; Hyg. Astr. 4, 14; Macr. S. 2, 1, 22.