Lewis & Short

Mĕgălŏpŏlis, is, f., or Mĕgălē pŏ-lis (acc. Mĕgălēn pŏlin), f., = Μεγαλόπολις or Μεγάλη πόλις, a city of Arcadia, the birthplace of Polybius.
Form Megalopolis, Liv. 36, 31, 6; 45, 28, 4.
Form Megale polis, Plin. 4, 6, 10, § 20; Liv. 32, 5, 5; 35, 36, 10.
Hence,

  1. A. Mĕgălŏpŏlītae, ārum, m., the inhabitants of Megalopolis, Liv. 28, 8.
  2. B. Mĕgălŏpŏlītānus, a, um, adj., of or belonging to Megalopolis, Megalopolitan, Liv. 36, 13.
    Subst.: Mĕ-gălŏpŏlītāni, ōrum, m., the inhabitants of Megalopolis, Liv. 32, 22.