These are the vertebrates that possess hair and mammary glands.
Mammals are endothermic.
They show the presence of external ear called pinna.
The development of teeth occurs in the jaw socket (thecodont); teeth can be classified as incisors, canines, molars and premolars (heterodont); the milk teeth are replaced by permanent teeth (diphyodont).
They have a four chambered heart with double circulation.
Respiratory, excretory, nervous and reproductive systems are highly developed.