from the GNU version of the Collaborative International Dictionary of English
- n.pl. A group of Mammalia, including the marsupials and monotremes; -- so called because the corpus callosum is rudimentary.
from The Century Dictionary and Cyclopedia
- In Owen's classification, the lowest of four subclasses of Mammalia, including the marsupials and monotremes, or Didelphia and Ornithodelphia; the implacental mammals.
Sorry, no etymologies found.