The Collaborative International Dictionary of English v.0.48:

Peritonaeum \Per`i*to*n[ae]"um\, n. (Anat.)
   Same as Peritoneum.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Peritoneum \Per`i*to*ne"um\, n. [L. peritoneum, peritonaeum, Gr.
   ?, ?, fr. ? to stretch all around or over; peri` around + ?
   to stretch.] (Anat.)
   The smooth serous membrane which lines the cavity of the
   abdomen, or the whole body cavity when there is no diaphragm,
   and, turning back, surrounds the viscera, forming a closed,
   or nearly closed, sac. [Written also periton[ae]um.]
   [1913 Webster]