What are the three subclasses of mammals?

There are three infraclasses of mammals, not subclasses:

1. Prototheria: This infraclass contains the monotremes, which are egg-laying mammals like the platypus and echidnas.

2. Metatheria: This infraclass contains the marsupials, which are mammals that give birth to relatively undeveloped young that complete their development in a pouch. Examples include kangaroos, koalas, and opossums.

3. Eutheria: This infraclass contains the placental mammals, which are mammals that give birth to relatively developed young that have been nourished in the mother's uterus through a placenta. Examples include humans, cats, dogs, and elephants.