Monotremes:

The monotremes can be distinguished from the placnetal mammals (or eutherian mammals) on the way they reproduce.  Monotremes are the only  mammals are oviparous (egg layers).

There are only a few types of monotremes: the Duck-billed platypus, the short-billed echidna and the long-billed echidna.

The name Monotremata means "one opening", and refers to the cloaca, a common opening for the fecal, urinary and reproductive tracts.