Monotremes, by the way, are a more primitive form of mammal than marsupials. Here's the way it works...

Monotremes - have mammary glands, but no nipples: only holes from which milk oozes, and no placentas.

Marsupials - have mammary glands and nipples, but no placentas.

Placental mammals - have mammary glands and nipples and placentas.


The order of most primitive to least primitive (mammals) goes like this...

Monotreme
Marsupial
Placental mammal