Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Mărĭandȳni, ōrum, m., = Μαριανδυνοί, a people of Bithynia, Mel. 1, 19, 1; 7; 2, 7, 2; Val. Fl. 4, 171; Avien. Descr. Orbis, 960; Mart. Cap. 9, § 924.
Hence, Mărĭ-andȳnus, a, um, adj., of or belonging to the Mariandyni, Mariandynian: sinus, Plin. 6, 1, 1, § 4: arenae, Val. Fl. 4, 733.