Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† pĕristȳlĭum, ii, n., = περιστύλιον, a place surrounded with columns on the inside, a peristyle, Vitr. 3, 1; 5, 11; Plin. Ep. 10, 70, 3; 10, 71, 1; Suet. Aug. 82; cf. peristylum.