The two types of mammals are placental mammals and nonplacental mammals. Placental mammals are humans, dogs, apes, etc. Nonplacental mammals are kangaroos, platypus, opossum,etc.
The echidna and the platypus are non-placental mammals. They are monotremes, or egg-laying mammals. Kangaroos, wallabies, koalas, bandicoots, wombats and Tasmanian devils are just a few other...
Mammals can be split into two subclasses, the prototheria, which includes the egg layers such as the platypus, and the theria, which includes all the placentals mammals and live-bearing marsupials....
There are three types of mammals existent in the world. Arranged from most primitive to most advanced these are: Monotremes (represented by the Platypus and Echidna): These are egg laying mammals...