Air-breathing warm-blooded Vertebrata in which the epidermis de-velopes hairs over a greater or less extent of the surface of the body: which are viviparous with the exception of Prototheria, and always nourish their young for longer or shorter periods after birth with the secretion of lacteal or mammary glands. There are two occipital condyles to the skull and seven cervical vertebrae. The coelome is divided into a thoracic and abdominal portion by a muscular diaphragm. The aorta is single and bends over the left bronchus; and the red corpuscles or haematids are non-nucleated. The rectal and urogenital apertures are typically separate, and the former placed behind the latter.

The minute structure and the character of the hairs vary much in different Mammals, and on different parts of the body in the same Mammal. The hairy coat is scanty in some instances, e. g. Sirenia, Hippopotamus; and in Cetacea is restricted to the snout, where it may be present only in the foetus. The edges of the eyelids are protected by a single row of hairs - the eye-lashes or cilia: and the snout in most Mammalia has a few stout tactile hairs, or vibrissae, connected with an abundant nerve-supply. Certain hairs, e. g., those of the mane and tail in Equidae, the vibrissae, appear to be persistent, but as a rule the coat is shed both before and after the winter season: the winter coat being thicker and sometimes, as in Arctic Mammalia, white in colour. Epidermic structures in the shape of nails, claws', or hoofs, protect the terminal joints of the hand and foot, except in Cetacea and certain fingers in the Chiroptera. Other instances of epidermic skeletal structures are the imbricated scales covering the body and tail in Manis among Edentata, or the under side of the tail in certain flying Rodents (Anomalurus); the flat scales placed edge to edge on the tails of some Rodents, e. g. the Beaver (Castor), of certain Insectivora (Ptilocercus) and Metatheria; the spines of some Insectivora (the Hedge-hog, Erinaceus), Rodentia (the Porcupines, Hystricidae), and of Echidna among Prototheria; the horns as opposed to their bony supports, the horn-cores, of hollow-horned Ruminantia (Cows, Sheep, Goats, Antelopes); the nasal horns of the Rhinoceros; the thickened epidermis of the callosities, hair-less patches of skin occurring in different regions of the body, e. g. over the ischial tuberosities in Apes; and the whale-bone plates implanted in the gum of the Baleen Whales. A bony dermal skeleton is found only in the Dasypodidae and the extinct Glyptodontidae among Edentata in the shape of scutes or plates covered by a thickened epidermis.

The antlers, or bony frontal outgrowths, of the Deer, which are shed and renewed annually, must be reckoned under the same head. The glands of the integument are the following: sebaceous glands connected with the roots of the hairs; sudoriparous glands opening on the surface of the skin, and rarely wanting, e. g. in Cetacea, as well as certain other special glands, e. g. the Meibomian glands of the eyelids, the lacrymal glands; and glands known as suborbital, anal, inguinal, etc, according to position. The mammary glands characteristic of the class, which secrete the milk, are probably modified skin-glands. Their ducts open on an area which is usually raised into a more or less prominent papilla - the true teat. In Ungulata, however, this area lies at the bottom of a tubular depression produced by the growth of a surrounding wall forming a false teat. The number of teats present varies according to the number of young usually produced at a birth, from two, e. g. in the Primates, to twenty-two as a maximum in Centetes among Insectivora. When numerous they usually extend in two rows from the pectoral to the inguinal region: when few they are restricted to the pectoral, abdominal or inguinal regions.

In all Mammalia except the Hare, Lepus timidus, a layer of adipose tissue, the panniculus adiposus, sometimes attaining great thickness as in the blubber of the Cetacea and Seals, is interposed beneath the skin and underlying muscles and bones; and in most instances there is a well developed system of skin-muscles.

The bones of the skull, with the exception of the lower jaw, the auditory ossicles, and hyoid, are united by sutures which persist as a rule. The two occipital condyles are formed by the exoccipital bones, but in some Mammals in part by them, in part by the basi-occipital. The prae-maxillary, maxillary and palatine bones possess palatal plates which constitute the hard palate and separate the narial and buccal cavities. There are distinct lacrymal and tympanic bones and the latter is often dilated into a tympanic bulla and prolonged outwards as an external bony meatus. The periotic bones anchylose inter se, and form a compact periotic mass which is either connected by suture to neighbouring bones or is partially or wholly free. The carotids enter the cranial cavity, either by piercing the periotic mass or passing between it and the base of the skull. The rami of the lower jaw articulate directly with the squamosal bones. Each ramus consists in the adult of a single bone which unites with its fellow at the mental symphysis either by suture or by anchylosis. It is derived in Man from four centres of ossification which correspond respectively to a mento-meckelian, dentary, splenial and coronoid element.

There are three auditory ossicles, a malleus, incus and stapes, which represent, the first-named, the articular element of the lower jaw, the second, the quadrate bone, and the third the columella auris of the Sauropsida and the hyoman-dibular of Fish. The cervical vertebrae are reduced to six in the Manatee, in Choloepus Hoffmanni among Edentata, and increased to nine in Bradypus tridactylus in the Order named. Cervical ribs are represented only by centres of ossification often absent. The rib-element is often wanting in the seventh cervical vertebra; it is sometimes present abnormally, however, as a free rib. The cervical is always sharply marked off from the dorso-lumbar series of vertebrae. The latter series varies in number between the extremes of 14 in the Armadillo (Edentata) and 30 in Hyrax. The number is often constant within the limits of a given group, e. g. to 19 in Artio-dactyla among Ungulata. The dorsals are usually 12 or 13. There may be but a single sacral vertebra as in Perameles among Metatheria, or more commonly two; and the number is generally increased by the anchylosis of a variable number of anterior caudal vertebrae, more rarely of a posterior lumbar.