Theria (from the Greek θηρίον, wild beast) 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 The subclass includes humans. They 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 curren… (More on Theria)