"Marsupials are a primitive as compared to placental mammals. Which is another way of saying that marsupials are a primitive form of mammal"

Sorry - one last thing.

That's a misunderstanding CarolC; marsupials are not primitive mammals, they're just different. Placental mammals didn't evolve from marsupials, the fossil evidence indicates they evolved alongside them towards the end of the Mesozoic.

Monotremes, the third family of mammal (like the Platypus) were also though of as primitive but actually branched out from the mammalian line earlier and are as 'advanced' as us arrogant humans like to think are.

No more or less primitive.