MAMMALIA, a term invented by Linnaeus (1758) to include that class of animals in which the young are brought forth alive and nourished with milk from the mother's breasts (mammae). Typical examples are the dog, cow, rabbit, monkey, man. Whales are also mammals, although externally fish-like in appearance. Hair (q.v.) is a typical mammalian structure. Mammals, like birds, are "warm-blooded" or homoeothermal animals, that is, they differ from "cold-blooded" or poecilothermal forms, such as fishes, amphibians and reptiles, in their superior ability to regulate their own body temperature, so that in spite of wide variations in the temperature of the surrounding medium the body tempera ture of a typical mammal is maintained in health at a high level and within the relatively narrow limits that are best adapted to the needs of the animal. Some of the most characteristic features of mammals are the expression of the relatively high pace main tained by the vital processes. Here may be mentioned the flexible skin, together with the sweat glands and sebaceous glands, the diaphragm, the complex lungs and heart.
The origin of the mammals lies among the extinct mammal-like reptiles of the Triassic age. The subject may be divided into the following headings, I. Nutrition; II. Locomotion; III. Control Systems ; IV. Reproduction ; V. Rise of the Mammalian Orders; VI. Continental Dispersal of Orders.
Mammals are distinguished from lower vertebrates by the fact that the mandible, consisting on each side of a single element (corresponding with the dentary bone of lower vertebrates), articulates directly with the squamosal without the intervention of the quadrate and articular which form the functional mandibu lar joint in the lower vertebrates. Early embryos of man, and
many other mammals, show the dentary of each side forming on the outer side of the stout cartilaginous lower jaw, which corre sponds with the Meckel's cartilage or core of the lower jaw of lower vertebrates. The back of this in the embryo mammal ends in a well-developed joint corresponding to the joint between the quadrate and articular elements of the lower vertebrates. As development proceeds the dentary grows upward and establishes a new joint with the squamosal bone of the skull, while the old reptilian joint dwindles in size, pulls away from the dentary bone, and gives rise to the joint between the malleus and incus of the middle ear, which are thus vestiges of the articular and quadrate of lower vertebrates.
In the higher mammal-like reptiles of the Triassic of Russia and South Africa the adult dentition was already differentiated into four kinds of teeth : incisors, canines, premolar, and molars; the premolars and molars were cuspidate and in some genera had acquired accessory basal spurs and additional cusps; moreover, the teeth were set in sockets and in some the cheek teeth show an incipient division of the single roots, while the dentition as a whole was reduced to two sets corresponding to the deciduous and per manent dentitions of mammals.