Lewis Short
(adjective) : pentăgōnĭus, a, um, , = πενταγώνιος and pentăgōnus, a, um, = πεντάγωνος
* Quinquangular, pentagonal.— Subst.: pentăgōnĭum, i, n., a pentagon (post-class.): pentagonii ratio, Auct. de Limit. p. 257 Goes.—pentăgōnum, i, n., for pentaphyllon, App. Herb. 2.
Charlton T. Lewis, Charles Short, A Latin Dictionary