The mammals, as compared with reptiles, are characterized by a simplified and reinforced skull with two occipital condyles, which joint with heavily modified first cervical vertebra. Their lower jaw consists of a single dental bone articulating with the squamosal bone of the axial skull. The body is covered with hares. The body temperature is more or less constant. Four-chambered heart is totally divided into venous (right) and arterial (left) halves. Red blood cells are flat, round, without nucleus in mature state. The hearing apparatus consists of an outer, middle and inner parts; the pinna is well developed in most of the taxa. The oral cavity is separated by the secondary bony palate from the nose cavity, in which there are usually sophisticatedly convoluted nasal shells with olfactory epithelium.
The teeth (absent in some edentates and baleen whales) are in alveoli. Dentition is differentiated into incisors, canines, premolars and molars (except for toothed whales). All teeth have single generation, except for molars, the latter have two of them.
There are usually 7 cervical vertebrae, and very rare 6 (some sea cows) or 8-9 (some sloths). Limbs initially (primitively) are five-fingered, but in different orders they are significantly changed due to course of adaptations to different ways of locomotion — on the solid ground, on the trunks and branches of trees, in the air and water, and underground.
Cerebral hemispheres are highly developed, their cortex concentrates neuronal structures involved in the most important mental functions.
The skin is made of a highly developed connective tissue layer (subcutaneous) and epidermis with numerous secondary formations, including hair with their derivatives. There is a lot of various specific skin glands including sweat and milk ones.
Mammals are highly flexible and diverse ecologically. They live everywhere except for the deep marine waters, solid ice land areas (Central Greenland, Antarctica), and the snow mountain peaks above 5000 m. Particular mammalian orders are adapted to quite different modes of life: there are terrestrial, underground, arboreal and aquatic forms among them. One of the orders — chiropteran — perfectly mastered flapping flight, cetaceans and sirens are fully aquatic animals having lost an ability to go out to the land. Ecological flexibility of some species is very large (rat), while others are strictly specialized (for instance, the koala eats only certain types of eucalyptus).
Means of dispersion in terrestrial mammals are rather limited (they can not themselves cross the seas), therefore they are good indicators of zoogeographical areas recognized to a large extent by just the data on their distribution.
Distribution ranges of mammals bear clear traces of the historical (geological) changes in the Earth's surface (for instance, former intercontinental connections). This, together with relatively extensive paleontological data, provides important source for study of the history of the Earth.The economic importance of mammals is very large: this class includes most of the domestic and game animals. Many small mammals, especially rodents, are pests damaging agricultural or human constructions and communication means. Man, as a representative of this class, has a global and irreversible effects on the entire biosphere of the Earth.