Class Mammalia is a vertebrate class, which has the following characteristics: The skin is covered by epidermal hair at some stage of the animal's development (except in aquatic forms) and contains sebaceous and sweat (sudoriparous) glands.
Mammals are viviparous (except the oviparous order Prototheria) and have the habit of nursing their young with secretions of the female's mammary glands.
As an exception to all other vertebrates, mammals have a lower jaw (mandible) that consists of a single pair of dentary bone (dentaries) alone, which articulates directly with the squamosal (cheekbone).
Mammals have two sets of teeth; deciduous or milk teeth and permanent teeth. The shapes of the teeth are different to perform different functions. A double occipital condyle is present.
The external ear is well-developed. The middle ear consists of an articulated chain of three auditory ossicles: maleus, incus and stapes. The stapes equals the whole columellar apparatus of a lower tetrapod. The other two hearing bones are homologous with the articular and quadrate bones, which form the jaw joint of lower vertebrates. Because mammals evolved a different jaw articulation, the former joint elements were given a new use in the middle ear.
The vertebrae and long bones have three centers of ossification.
The thoracic and the abdominal cavities are divided by a fully muscular diaphragm and respiration is by lungs. The mammals are homoiothermous. Their red blood cells lack a nucleus. They have a double circulation, where the left aortic arch empties the left ventricle, and there is no right aortic.
The brain has characteristic large cerebral hemispheres.
The Mammalia are found all over the world except for most of Antarctica. Some mammals are terrestrial, some burrow beneath the Earth's surface (fossorial), some swim in water (aquatic, including marine), some climb into trees (arboreal), or some fly through the air (aerial).
Most mammals are at least somewhat omnivorous, but many are primarily carnivorous or herbivorous; some have highly specialized diets, while others are very general in food selection. Social aggregations vary from individuals which live alone to herds or colonies of up to millions of individuals.
The class Mammalia includes 2 extant subclasses: Subclass Prototheria (primitive mammals characterized among others by a small alisphenoid bone and no trituberculate teeth) with 1 extant order (Order Monotremata, the duckbill and spiny anteater) and subclass Theria with 21 extant orders.