Definitions

Subclass Marsupials are mammals that raise their young in a pouch. This subclass includes: koalas, kangaroos, possums, koalas, and wombats.

Subclass Monotremes are mammls that lay eggs. This subclass consists of the Echidna and platypus.

Subclass Placentals are mammals that give birth to very well-developed young. Examples include: deer, goats, dogs, cows, and humans.