• (n. pl.) A group of Mammalia, including the marsupials and monotremes; -- so called because the corpus callosum is rudimentary.
Ly`en·ceph'a·la noun plural [ New Latin , from Greek ... to loose + ... the brain.] (Zoology) A group of Mammalia, including the marsupials and monotremes; -- so called because the corpus callosum is rudimentary.