A subclass of primitive mammals, which probably left the main stock in the Mesozoic, and are found only in Australasia. The adults have no teeth, the cervical vertebrae bear ribs, the limbs are held laterally, the shoulder girdle has precoracoids and an inter-clavicle, and large yolky eggs are laid. The young are fed after hatching on milk produced by specialized sweat glands, whose ducts do not unite to open on nipples. One order, the Mono- tremata. Duck-billed Platypus, Spiny Anteater.
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