Final answer:
The platypus and echidna are the only living forms of monotreme mammals. Monotremes are unique because they lay eggs instead of giving birth to live young. They are found in Australia and New Guinea.
Step-by-step explanation:
The platypus and echidna are the only living forms of monotreme mammals. Monotremes are unique among mammals because they lay eggs, rather than giving birth to live young. The platypus and echidnas belong to the order Monotremata and are found solely in Australia and New Guinea. Monotremes have a leathery shell, similar to reptile eggs, and retain their eggs through about two-thirds of the developmental period before laying them in nests.