Are platypuses some sort of mammal reptile intermediate?

Uhh, no. They're monotremes, not synapsids. Monotremata is an order of Mammalia. So they're full mammals. The reason they lay eggs is because reptiles, mammals, and birds are amniotes. That means our eggs all share common characteristics (see here a href="http://tinyurl.com/2ym8j5)." rel="nofollow"http://tinyurl.com/2ym8j5)./a This is to be distinguised from an amphibian or fish egg. Whether the egg is hard or not depends on if the creature is viviparous or oviparous. Monotremes are simply oviparous mammals.