What is a non-placental mammal?

A mammal is an animal that gives birth to live young and suckles them. A non-mammal is an animal that is born in an egg and does not drink milk from its mother. For example, a bird is a non-mammal because it is born from an egg and does not drink milk. But, there are some exceptions. The duck-billed platypus is one. The platypus is a mammal that suckles its young, but lays eggs!