Lewis & Short

Theumēsus, i, m., a mountain of Bœotia, near Thebes, Stat. Th. 4, 372.
Hence, Theumēsius, a, um, adj., Theban, amnis, i. e. Ismenus, Stat. Th. 8, 462: arva, id. ib. 2, 383: juvenis, i. e. Polynices, id. ib. 2, 331.