What is the only mammal that lays eggs?

According to the University of California at Berkeley, the duck-billed platypus and four species of spiny anteaters are the only mammals that lay eggs. All of them live in New Guinea and Australia.

Both the platypus and the anteaters are classified as monotremes because of their egg-laying reproductive method. In other ways, they are similar to mammals in that they have a lower jaw with a single bone and three bones in the middle ear. They also rapidly burn calories. In addition, they are hairy and produce milk to feed their young. Although they have no teeth, their snouts or beaks are modified to suit to their food-seeking style.