lyencephala

from The Collaborative International Dictionary of English v.0.48

Lyencephala \Ly`en*ceph"a*la\, n. pl. [NL., fr. Gr. ? to loose +
   ? the brain.] (Zool.)
   A group of Mammalia, including the marsupials and monotremes;
   -- so called because the corpus callosum is rudimentary.
   [1913 Webster]