Describe class - Mammalia. 

Explanation

Mammalia are found in a variety of habitats - polar ice caps, deserts, mountains, forests, grasslands and dark caves.
Some of them have adapted to fly or live in water.
The most unique mammalian characteristic is the presence of milk producing glands (mammary glands) by which the young ones are nourished.
They have two pairs of limbs, adapted for walking, running, climbing, burrowing, swimming or flying
The skin of mammals is unique in possessing hair.
External ears or pinnae are present.
Different types of teeth are present in the jaw.
Heart is four-chambered. 
They are homoiothermous.
Respiration is by lungs.
Sexes are separate and fertilisation is internal.
They are viviparous with few exceptions and development is direct.
Examples : Oviparous - Ornithorhynchus (Platypus),
Viviparous - Macropus (Kangaroo), Pteropus (Flying fox), Camelus (Camel), Macaca (Monkey), Rattus (Rat), Canis (Dog), Felis (Cat), Elephas (Elephant), Equus (Horse), Delphinus (Common dolphin), Balaenoptera (Blue whale), Panthera tigris (Tiger), Panthera leo (Lion).