Monotremes are mammals that lay eggs. Platypi are monotremes. (If you like, you can change the definition of “mammal” to say “give birth to live young, except

Monotremes are mammals that lay eggs. Platypi are monotremes. (If you like, you can change the definition of “mammal” to say “give birth to live young, except for monotremes”. That's exhaustive.)