Platypus is not mammal.
It's a monotreme. They have a cloaca, rather than an anus and a genital opening.
That puts it, alone with the echidna, alongside all the Mammalia, Reptilia, Gastropoda, Insecta, etc. etc.
See:
http://lib.bioinfo.pl/meid:289052
and
http://en.wikipedia.org/wiki/Monotreme

John