It is probable that the single set of teeth of Monophyodonts corresponds to the permanent set in Diphyodonts. The milk dentition includes incisors, canines, and deciduous (prae)molars which are replaced by corresponding teeth in the permanent set as a whole or in part. The milk dentition is sometimes shed in utero (Guinea-pig), absorbed at an early period without cutting the gum (Seal), or aborted altogether (e. g. Rat and some other Rodents). The teeth are composed of dentine capped above the gum by enamel, partially or completely, and in the root or fang coated with cement. The exposed portion, or crown, varies much in character, and when folded the interval between the folds is often filled with cement. Enamel is entirely absent in Edentata and the Dugong among Sirenia. Dentine with vascular channels (vaso dentine) makes up the whole tooth in most Edentata; and is found in the Manatee (Sirenia) and Tapir (Ungulata). The pulp sometimes grows throughout life, e. g. incisors of Rodents, teeth of Edentata; or up to a certain age, e.g. molars of Horse and some Rodents; and in Cetacea it either atrophies or is calcified.
The mouth has soft fleshy lips, except in Cetacea and Prototheria. The tongue is well developed, is rich in glands, in tactile papillae, and possesses special gustatory bulbs lodged at the sides of certain papillae or folds. There is a soft palate or velum pendulum palati. Three pairs of glands - parotid, submaxillary, and sub-lingual - open into the oral cavity. They are wanting in Cetacea, and much reduced in Seals. The labial glands are small in size. The stomach is always sharply marked off from the oesophagus, and is generally of complicated structure in vegetable feeders. The small intestine is of considerable length, and is as a rule separated from the large intestine by an ileocaecal valve, rarely wanting as in some Carni-vora. The caecum is sometimes absent, e. g. some Carnivora, and is of great size in most herbivorous Mammals. It is small, and double only in the two-toed Sloth among Edentata. The large intestine (colon) is of great relative length: the rectum is very short, and the anus is posterior, i. e. dorsal to the urogenital aperture. The whole tract is remarkably rich in glands, and the small intestine possesses innumerable minute villi. The liver is often complex. The ligamentum teres, or remnant of the umbilical vein, divides it into a right and left lobe.
The latter is sometimes subdivided into a central and lateral lobe. The right lobe is usually subdivided in a similar manner, and often has a caudate lobe cut off from the lateral lobe and fitting on the right kidney and a Spigelian lobe projecting behind the entrance of the portal vein. A gall-bladder is rarely absent (Horse, Cetacea, some Rodents) and is always a diverticulum of the hepatic duct. The pancreas is usually compact, but is a scattered gland in some Rodents, e. g. Rabbit, Rat, and its duct as a rule unites with the hepatic duct to form a ductus communis choledochus.
The red-blood corpuscles are biconcave discs, circular in outline, elliptical in the Camel and Llama. Their size varies much, and in one and the same group are largest in the larger species. They are largest in the Elephant (1/2745"), some Cetacea and Edentata, smallest in the Tragulidae among Ungulata (1/12325").The heart has two auricles and two ventricles. The right auricle has no sinus venosus, and the auricular septum is marked by a fossa ovalis, indicating the position of the foramen ovale in the foetus by which blood passed from the right to the left auricle. The right, like the left, auriculo-ventricular valve is membranous, and consists of three flaps (hence tricuspid), one of which, the septal flap, is attached to the septum of the ventricles. The aorta crosses the left bronchus, and the right subclavian artery represents the corresponding, i. e. the fourth aortic arch of the right side. The sub-clavians and carotids arise from the aortic arch in various ways. The blood of the left vena cava superior is frequently carried across to the right cava superior by a transverse vein, and the cardiac end of the vessel (= coronary sinus) alone persists receiving the coronary veins of the heart, and sometimes, as in many Ungulata, a left azygos vein.
The veins of the extremities at least possess valves, structures wanting, however, in Cetacea. The lymphatic system is well developed, and there are numerous lymphatic glands. The thoracic duct opens into the left subclavian vein. The lymphatic vessels have valves. The tonsils, masses of adenoid tissue at the entrance to the pharynx, are peculiar to Mammals. For the hibernating gland, see pp. 2 and 4.
The nostrils which lead to the air-passages are double, except in some toothed Whales. The entrance to the trachea is protected by an epiglottis. There is a larynx composed of two arytenoid cartilages, a single thyroid and cricoid cartilage, and provided with well-developed muscles. The cartilage rings of the trachea are incomplete, posteriorly or dorsally. The trachea divides into two principal bronchi. There is but a single bronchia rising above the entrance of the pulmonary artery. This eparterial bronchia is present either on both sides or on the right only, e. g. Primates, and in some instances it rises on the right side from the trachea, e. g. in Artiodactyle Ungulata. It is wanting in the Porcupine (Hystrix). There are 9 hyparterial bronchia. The lungs are suspended freely in pleural sacs, and they are frequently lobed, especially on the right side. The respiratory capillaries are distributed on the walls of the lobed air-cells, or alveoli, in which the ultimate branches of the bronchiae terminate.
The kidneys sometimes retain the embryonic lobed character (Cetacea, the Seals, Bear, etc, among Carnivora), but, as a rule, the lobes fuse completely at their outer extremity, while at their inner they unite to form the Malpighian pyramids, upon which the tubuli uriniferi open. These pyramids project into the dilated upper ends or pelves of the ureters. The blood-vessels of the gland and of the surrounding adipose tissue anastomose, but there is no functional renal-portal system. The ureters enter the dorsal aspect of the urinary bladder, either near its apex or more usually its fundus ( = base). This bladder is a remnant of the urachus, or internal portion of the allantois. It has an outlet or urethra which, in the male, unites with the genital ducts to form a well developed uro-genital canal, while in the female the urethra and genital outlet alike fall into a shallow depression, the vestibule which represents the urogenital canal. In some female Rodentia, Insectivora and Lemurs, the female urethra perforates the clitoris (infra), and is entirely separate from the genital outlet. The testes may be retained within the abdomen, e. g. Elephant, or pass at the breeding season into a temporary scrotum, e. g. some Rodents, or be lodged permanently in a scrotum.
The termination of the vasa deferentia are often furnished with vesiculae seminales. Prostatic and Cowper's glands often open into the urogenital canal which enters an erectile intromittent organ or penis, composed of two corpora cavernosa, and of a corpus spongiosum inclosing the canal and forming a terminal glans. The two former bodies are attached to the ischia, except in the Sloths and Ant-eaters, among Edentata, in the Meta- and Proto-theria, and the latter is absent in the Edentata just named. The ovaries are relatively small: the oviducts have wide abdominal apertures, usually fringed ( = fimbriate), and are differentiated into an oviducal portion ( = Fallopian tube), a muscular uterine portion, and a canal or vagina. But the characters of these parts vary in the three chief divisions of Mammalia. The penis of the male is represented in the female by the clitoris. The ovum is relatively small: the granulosa cells are very numerous, and, during the growth of the Graafian follicle, a liquid - the liquor folliculi - appears in the centre of the follicle, while the granulosa cells occupy the periphery, and bear the ovum within a small projection, or discus proligerus.
Segmentation is total, but slightly irregular.
The class Mammalia is divisible into three sub-classes - Eutheria, Metatheria, and Prototheria - .which are the equivalents of De Blainville's three groups - Monodelphia, Didelphia, and Ornithodelphia.
'Mammalia,' Flower, Encyclopaedia Brit. (ed. ix.) xv; Giebel and Leche Bronn's Klass. und Ordnungen des Thierreichs vi, Abth. v. (in progress); Huxley, P. Z. S. 1880. Nails, Claws, Hoofs, Boas, M. J. ix. 1883; Gegenbaur, M. J. x. 1885. Types of molar Teeth, Cope, Journal Acad. Nat. Sc. Philadelphia, viii. 1874-81. Mammary glands, Klaatsch, M. J. ix. 1883. Sublingua, Gegenbaur, M. J. ix. 1883. Tongue, Id. M. J. xi. 1886. Osteology of Mammalia, Flower, ed. 3 by Gadow, 1885.
Mammalian Descent, W. K. Parker (Hunterian Lectures), London, 1884. Mammals in relation to primaeval Times, O. Schmidt, Internat. Ser. liv. 1885.