The Monotremata are a subclass of mammals, living members of which are restricted to five species – four species of echidna (spiny anteater) and the duck-billed platypus, all found only in Australia and New Guinea. Monotremes differ from all other mammals in not bearing live young because, like reptiles, they lay eggs. The eggs are temporarily transferred to a pouch beneath the female's abdomen where they hatch and are nourished by rudimentary mammary glands.
Monotremes have highly modified snouts or beaks, and modern adult monotremes have no teeth. Like other mammals, however, monotremes have a single bone in their lower jaw, three middle ear bones, high metabolic rates, and hair, and they produce milk to nourish the young.
The echidna (see diagram) lays a tiny egg (1) that is soft-shelled and resembles a reptile's egg. Once the egg is laid, the echidna uses its hind limbs to roll it to a special incubation groove (2). The minute hatchling is about 1.25 centimeters (0.5 inch) long.