I'm not a professional naturologist. I just care about how they taste. My previous post was tongue in cheek.
I do, however, invite you to present your case.
The monotremes, represented by the platypus and two species of echidna, occur only in Australia and New Guinea.
They're a very odd set of animals. Both lay eggs, rather than bearing their young alive. However, their young are fed with milk from mammary glands, so they're mammals...the only egg-laying mammals.
Since their home ground was isolated from Africa a long time ago, they've developed in their own way due to that isolation. There's a very good fossil history now for the monotremata, and it's fascinating.
If you look at the skull of an echidna, and didn't know that it was a mammal, you'd be almost certain that it was a bird's skull. The platypus, too has a skull that doesn't look much like a mammals, either.
In one of the oddest things about the monotremata, in my mind, is the fact that the echidnas carry their eggs, after they're laid, in a small pouch until they hatch. Then, the young attach themselves to a nipple.
This is pretty familiar stuff, since the marsupials also have a pouch, but their young are born alive, but in a very early fetal stage, and live in the pouch, attached to a nipple, just like the echidnas.
The monotremata are a relic, with only three living members of the group, and they're probably going to go extinct before another couple hundred years pass. The later marsupial species are much more varied, however, and are doing OK.
The whole mammalian ecology of Australia is quite a study for evolution researchers. It's fascinating.
If you need more information, just search for "monotremes" on Google. There are several excellent sites.