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Monotremes are one of the three main groups of living mammals, along with placentals (Eutheria) and marsupials (Metatheria). The monotremes are typified by laying eggs rather than bearing live young, and by structural differences in their brains, jaws, digestive and reproductive tracts, and other body parts compared to the more common mammalian types. However, like all mammals, the female monotremes nurse their young with milk.

(Source: Wikipedia)
Platypus - Ornithorhynchus anatinusPlatypus - Ornithorhynchus anatinusShort-beaked Echidna - Tachyglossus aculeatus