Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Messēis, ĭdis, f., = Μεσσηίς, a fountain in Thessaly, Plin. 4, 8, 15, § 30.
As adj., of or belonging to the fountain Messēis: Messeides undae, Val. Fl. 4, 374.