Echidnas and Platypus
The order Monotremata consists of the egg laying mammals. This small mammalian order, consisting of only three members, the platypus and two echidnas (short-nosed and long-nosed echidnas) is considered to be the most primitive of all mammalian orders.
Monotreme resemble reptiles internally and differ from all other mammals in that they lay shell-covered eggs that are incubated and hatched outside of the body.
The skull of a monotreme has a smooth, rounded cranial portion ending in a long nose covered by rubbery sensitive skin. The sutures of the skull fuse and appear to fade as the animal matures. Young platypuses have teeth, but they do not emerge through the gums. True functional teeth are not present in adult monotremes.