Theria

Theria is a subclass of mammals that give birth to live young without using a shelled egg, including both eutherians (placental mammals) and metatherians (marsupials and their ancestors).

Extent
Members of this subclass have external ears, most can suckle on a nipple, and have an ankle specialized for power and range of motion. Therians are often classified by their specialized dentition.

Almost all currently extant (not extinct) mammals are therians. The only known exceptions are the platypus and the echidnas (spiny anteater), both of which are prototherian monotremes.