Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
amphĭthĕātrālis, e, adj. [amphitheatrum], of or pertaining to the amphitheatre, amphitheatrical: spectaculum, Plin. 11, 24, 28, § 84: magistri, Mart. 11, 70: pompa, Claud. Cons. Mall. Theod. 293.