a primitive mammal that lays large yolky eggs and has a common opening for the urogenital and digestive systems. Monotremes are now restricted to Australia and New Guinea, and comprise the platypus and the echidnas.
Order Monotremata and subclass Prototheria: two families
mid 19th century: from mono- 'single' + Greek trēma 'hole'
monotreme in other Oxford dictionaries
Definition of monotreme in the US English dictionary