The Collaborative International Dictionary of English v.0.48:

Pleuroperitoneum \Pleu`ro*per`i*to*ne"um\, n. [Pleuro- +
   peritoneum.] (Anat.)
   The pleural and peritoneal membranes, or the membrane lining
   the body cavity and covering the surface of the inclosed
   viscera; the peritoneum; -- used especially in the case of
   those animals in which the body cavity is not divided.
   [1913 Webster]

   Note: Peritoneum is now often used in the sense of
         pleuroperitoneum, the pleur[ae] being regarded as a
         part of the peritoneum, when the body cavity is
         undivided.
         [1913 Webster]