Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† pĕrĭtŏnaeum and pĕrĭtŏnēum, i, n., = περιτόναιον and περιτόνειον, the membrane enclosing the intestines contained in the abdomen, the peritoneum, Cael. Aur. Acut. 3, 17 (in Cels. 4, 1, and 7, 4, written as Greek).