edge-of-existence-edge:

The platypus was officlally classified as a mammal when it was discovered that it has mammary glands and it suckles its young.   However, much like the echidna, the platypus does not have nipples.  Instead, the milk flows from pores into grooves in the female’s abdomen, where the young can lap it up.

Another oddity about young platypodes is that they are born with teeth.  Aside from the egg tooth, used to help the hatchlings pierce the shell of their eggs, the baby platypus has three teeth in each of its upper jaws, a premolar and two molars.  It is unknown why these teeth are present, as they drop out before the babies leave the breeding burrow and never grow back; the adult platypus has horny plates in its mouth to grind food, and also swallows gravel to aid with digestion.