Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Ăcanthĭus, a, um, adj., from Acanthus, a town in Macedonia: sal, Plin. 31, 7, 41, § 85.
2. Ăcanthus, i, f., = Ἄκανθος, a town of Macedonia, now Erisso, Liv. 31, 45 fin.; Mel. 2, 2, 9; Plin. 4, 10, 17, § 38.