The Collaborative International Dictionary of English v.0.48:

paleencephalon \paleencephalon\ n.
   The more primitive parts of the brain phylogenetically; it
   includes most structures other than the cerebral cortex.

   Syn: paleoencephalon, palaeoencephalon.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

paleencephalon
    n 1: the more primitive parts of the brain phylogenetically;
         most structures other than the cerebral cortex [syn:
         paleencephalon, paleoencephalon, palaeencephalon]