Mammals are animals which are endothermic, have hair or fur, and have mammary glands which produce milk to nourish their young. Mammals are a Class of Chordate animals. Mammals are divided into three main groups depending on their reproduction style. Monotremes are primitive, egg-- laying mammals. Their eggs resemble reptile eggs. Their offspring lick the abdomen of the mother to obtain milk. Examples are duckbilled platypus and echnidnas. Marsupials are mammals that give birth to very immature embryos, which crawl into their mother's pouch, find a nipple and continue to feed and develop in her pouch. These are the kangaroos, koalas and other relatives found in Australia and New Zealand and oppossums which are the only North American marsupial. The final and most inclusive group of mammals is the Placentals. These are carried for a period of gestation in their mother's uterus and are nourished by the placenta until birth. An umbilical cord attaches the fetus to its mother's placenta. Placentals include bats, elephants, cattle, humans, dogs, to name a few.