Mammalia, the highest vertebrated animals, including man, warm-blooded, breathing by lungs separated from the abdominal cavity by a diaphragm, generally covered with hair, and bringing forth their young alive, which they nourish by the secretion of mammary glands (whence their name). Most mammals are commonly known as quadrupeds, from their having four feet suited for progression on a solid surface; but the terms are not synonymous, as most reptiles are four-footed, and the whales cannot be called quadrupeds. The form of mammals is very various; among them we see man walking erect, the flying bats, the swimming cetaceans, the bulky elephant, the slow-moving sloth, and the agile squirrel; yet the three regions of head, neck, and trunk can always be recognized in the skeleton, and generally in the living animal. The neck, though varying in length from that of man (one seventh of the spinal column) to that of the giralffe (three sevenths), with two or three exceptions, consists of 7 vertebrae; some of the sloths have 8 or 9, and some manatees are said to have 6 only; in the hoofed animals the length of the neck depends on that of the fore legs, for the purpose of grazing; but the elephant has a long proboscis to compensate for the shortness of the neck rendered necessary by the ponderous head; the extra vertebras of the sloths are by some considered as dorsals with rudimentary ribs to give additional mobility to the neck.

The number of dorsal vertebras varies from 11 in some of the bats to 22 in some of the sloths, man having 12; the lumbar vertebras, 5 in man, are 2 in the ornithorhynchus and 9 in some lemurs, stronger than the dorsals, and without ribs, which are replaced by long transverse processes; the sacral vertebras, usually 4, vary from 1 to 9; the rudimentary tail of man, the os coccygis, consists of 4 bones, but in the long-tailed manis there are 46 caudal vertebras. The skull is articulated to the spine by two occipital condyles, which permit the upward and downward motions of the head, the lateral and rotating movements depending on the articulation between the first and second vertebras; in whales the short neck is immovable as in fishes, and its bones are very thin and more or less consolidated together; the strong Ugamentum nucha, which supports the head, is attached to the spinous processes and skull. The caudal vertebras are of two kinds, one having a spinal canal, the other not, and the processes are always developed in accordance with the use made of the tail; in most mammals its movements are confined to brushing away insects from the skin but in the kangaroo it forms with the hind legs a firm tripod from which the animal , springs, and in some South American monkeys it is prehensile and used as a fifth hand in hanging from trees; in the whales it becomes a powerful swimming organ, is provided on its under surface with V-shaped bones for tin protection of the blood vessels, and, beina horizontal, is used principally as an organ by which to rise to the surface to breathe; in the beaver the transverse processesand the lower spinous are very large for the attachment of the muscles, which move the tail like a trowel chiefly in a downward direction.

The boms of the spine are united by elastic fibro-cartila-ges; these, in whales, form osseous disks, separating on maceration, and frequently used by arctic travellers for plates. - As all mammals breathe air, the mechanism of their respiration depends on the movable ribs and the diaphragm; man has 7 true and 5 false ribs, the former united to the sternum, the latter not; the number is in proportion to that of the dorsal vertebras; in the whale, of 12 ribs, 11 are false, in the unau 11 out of 23, in the ho: out of 18, in the cats 4 out of 13, and in the manatee 14 out of 16; in the carnivora they are dense and narrow, in the herbivora large, broad. and thick. The breast bone varies in shape according to the presence or absence of clavicles; in non-claviculated mammals the chest is compressed laterally, and the breast bone has a projecting keel as in birds; in hats it is much keeled, in the higher apes flat as in man. and in the moles it extends in front of the ribs, forming a distinct piece; in mammals with clavicles the chest approaches very nearly to that of man; the human chest, however, is the only one in which the transverse exceeds the anter-posterior diameter, causing the greater separation of the shoulders and the increased facility of movement of the arms.

The anterior ribs always extend as far as the breast bone, and are thus true ribs, differing in this respect from those of birds. Each of the rihs is usually connected by its head to an articular cavity formed by the bodies of two vertebrae, and by its tubercle to the transverse process of the posterior of the two; in the monotremata they are connected with the body alone, and in cetaceans often only with the transverse prooee The breast bone consists of several pieces, one behind the other, to which the anterior or true ribs are joined by cartilages which rarely become ossified; the posterior are the false or floating ribs, and are not attached immediately to the breast bone; this arrangement gives mobility to the chest and allows the elevation and depression of the ribs during respiration. The bones of the skull and face are immovably connected with each other, a character which does not occur in any of the lower classes; the brain cavity is larger than in birds and reptiles; the occipital condyles, near the centre of the base in man, are gradually removed to the posterior portion as we descend in the scale: the number of cranial bones, eight in man, is less than in most lower vertebrates.

For the general characters see Comparative Anatomy, where also arc given sufficient details on the organs of sense, teeth, digestive system, and bain covering. The lower jaw consists of two pieces, and is alone movable; in man it is sus-ceptible of motion up and down, laterally, and from before backward; in the carnivora the first movement, in the ruminants the second, and in the rodents the third, is specially provided for by the shape of the condyles and the form of the glenoid cavity. - The limbs of mammals van exceedingly in shape, according to the offices to be performed by them; we find the hand of man with its thumb opposable to the fingers, the four hands of the monkey, the paddles of the whale, the walking feet of the horse, the wing of the bat, the paw of the lion, the shovel of the mole, all constructed on the same type and modified from the same bony elements. The anterior limbs are always present, with a well developed scapular arch, usually kept in place by a clavicle; this last is present in man, monkeys, the insectivora, squirrels, and bats, but absent in cetaceans, the hoofed animals, and some edentates; in most carnivora and in some rodents it is imperfectly developed; it corresponds to the furcular bone in birds, and the nionotremata have in addition the second or coracoid clavicle of birds.

The shoulder blade is thin, Hat, and more or less triangular, generally with a well marked spine; it is long and narrow in herbi-vora. and placed perpendicularly on the anterior and Literal portion of the chest; in carnivora and rodent-, requiring more freedom of motion, it is oblique, and so of course is the glenoid cavity; jo.keys are well aware that an upright shoulder is the mark of a stumbling horse. The arm bone is nearly straight in man, much bent in the carnivora, long in monkeys and sloths, and short in ruminants and cetaceans; it is connected by a ball and socket joint with the scapula; below it articulates with the radius and ulna of the forearm by a hinge joint. The ulna is the longest in man and lies on the inside, and receives the arm bone in a deep sigmoid cavity; the radius is connected with the wrist, and turns with the movements of the hand, rolling around and upon the ulna; this independence of movement becomes less and less, acccording as the limbs are more used as instruments of progrexiion; in the carnivora and rodents the two bones are distinct, but the rotation is very imperfect, and in the hoofed animals generally the two make a single bone; the radius seems to form the principal bone, the ulna being frequently, as in the horse and bats, very rudimentary, The wrist in man consists of eight bones in two rows, in other mammals varying from live to eleven; to these, are attached the five parallel metacarpal bones in man, followed by the five fingers, each bavin- three joints, except the first or thumb, which has only two: in the ruminants the two metacarpals form the single cannon bone, sometimes with rudi-mentary bones on the side, as the splint bones of the horse; most pachyderms have three metacarpals, the elephant having five.

In animals which walk on the ends of the toes, the metacarpus is so lengthened that it has been mistaken for the forearm, and supposed therefore to be flexed in an opposite direction to that of man; but the lower part of the fore leg of a horse, for instance, is in reality the metacarpus, and the part called the knee is the wrist joint. The fingers vary from one to five; the third or middle finger is the most constant, and commonly the longest, and is the only one found in the horse; the thumb disappears first, then the little finger, and then the fourth finger; ruminants have the second and third, or fore and middle fingers. The hind limbs are more firmly connected to the trunk than the anterior; the supporting arch is the pelvis, composed of the ilium, ischium, and pubis on each side, the first joining the sacrum, the second forming the prominences upon which man sits, and the third uniting in front; in cetaceans there is only a rudiment of this bony arch, and the hind limbs are absent. The thigli bone, the longest in man, is in most other mammals relatively shorter; it is attached by a ball and socket joint to the pelvis, in man its axis being nearly that of the body, but in the lower mammals bending more and more forward until it forms an acute angle with the trunk.

The tibia and fibula correspond to the radius and ulna of the forearm, and have the patella or knee-pan in front of the articulation with the thigh bone; these are coalesced in various animals somewhat as are the radius and ulna; the tarsal bones correspond to the carpal, and are followed in the same manner by the metatarsus and toes. In the apes the great toe is opposable to the others, like the thumb, whence they are called quadrumana, four-handed.; while man rests his whole foot, from the heel to the toes, on the ground, other mammals walk chiefly on the toes; the horse stands on the tips of the middle fingers and toes, the heel being nearly as high up as the knee in man, the cat on the last two joints of several toes, and the bear on the metatarsus and toes; there is no animal, except man, that can bo properly said to touch the ground with the entire foot; in the seals all the bones of the leg and foot may be recognized, but they are united by a membranous web into a kind of caudal fin. The bones of mammals have not the air cells found in birds, but are either solid or their cavities are filled with an oily matter called marrow; there are, however, air cavities called sinuses, especially large in the frontal bone of ruminants, as in the ox and sheep, and greatly developed in the frontal region of the elephant; these communicate either with the nasal or auditory passages. - While most mammals resemble man in the arrangement of the muscles, others approach birds and even fishes in this respect; as they are less active than birds, their muscles are less firm and the tendons less liable to ossify; they are generally fewer in number than in man, and their variations from the human type are noticed chiefly in the limbs; in the mole, for instance, the flexors of the arm, the great pectoral, and the latissimus dorsi are very large the herbivora and pachyderms require massive muscles, and the agile carnivora compact and energetic ones; the muscles of the ears are specially developed in the herbivora, and those of the nose in the hog; the gluteus max-imus, the largest of all in man, is much smaller in the monkeys, and very small in the lower mammals; the nates in the horse are composed principally of the gluteus medius; the muscles of the calf, so characteristic of man, are small in all below him, and the short muscles of the human hand are absent in the lower mammals; those of the wings in bats are arranged somewhat as in birds, and those of cetaceans as in fishes.

A muscle remarkably developed in many mammals, but rudimentary in man, is the cutaneous layer, the pan-niculus camosas, of which the human analogue is the platysma myoides of the sides of the neck and face; we notice its action in the horse when a fly or any irritating object touches the skin, in the erection of the quills of the porcupine, and in the coiling of the body of the armadillo and hedgehog. The minute coccygeal muscles of man are represented by numerous and powerful ones in the prehensile tail of certain monkeys, in the strong trowel of the beaver, and in the fluke of the whale, analogous to the human multifidus spinm. - In man and mammals the heart is composed of two distinct halves, each divided into two cavities, an auricle and a ventricle; the course of the blood is from the left ventricle to the aorta and over the body, pure arterial; then traversing the systemic capillaries it enters into the veins, and is carried to the right auricle; thence it passes to the right ventricle, and thence by the pulmonary artery to the lungs, in whose capillaries it becomes purified by the oxygen of the respired air, and is returned by the pulmonary veins to the left auricle, whence it enters the left ventricle to be distributed as before.

Here, therefore, the blood passes twice through the heart and through two systems of capillaries before completing its circle; hence the circulation is called double, and it is also complete, as the whole mass of the blood is purified in the lungs before it is sent over the body. Before birth, when the lungs are impervious, the auricles communicate directly, and one or more vessels pass from the right ventricle to the aorta, conveying the blood over the body without sending it to the lungs; but when respiration begins these communications between the arterial and venous systems are closed. In the dugong the two ventricles are separated by a deep cleft; in some mammals the right auricle receives three venrc cavae; the apex is not inclined to the left, as in man, except in some monkeys, and in some hoofed animals two small flat bones are imbedded in the substance of the left ventricle. In cetaceans there is a plexiform arrangement of the arteries of the walls of the chest, allowing an ac cumulation of blood in them, to be used as re quired during prolonged submersion: in main ruminants the internal carotid forms a rete mirabile or network of vessels, at the entrance ot the skull, doubtless to prevent injury to tin bram from too great force of tin- blood while the head is in a dependent position; in the slow-moving sloths the arteries of tin- limb, communicate very freely, rendering compression during their climbing impossible except in a few vessels at a time.

A similar disposition prevails in the venous system; in the seal and otter, as in the ducks, the inferior cava is dilated into a receptacle which holds the blood while they are under water, and onh permits it to pass on to the lungs when they come to the surface; in the porpoise tortuous Him receive the intercostal veins, and in the foot of the horse a line network is distributed on the front of the coffin bone. The heart is composed of muscular fibres, each cavity having its own, arranged in a spiral manner from the point to the base; the course of the blood is directed from the auricles to the ventricles by the mitral valve on the left side and the tricuspid on the right, kept in place by tendinous cords attached to fleshy columns, and the entrances of the aorta and pulmonarj artery art-guarded each by three semilunar valves which prevent regurgitation. The lungs of mammals are almost always in pairs, and hang freely in the chest suspended by the straight windpipe, and enclosed within the serous cavity lined by the pleura; the air tubes are distributed to all their parts, and the pulmonary cells are minutely subdivided and do not communicate with any other air cells in the body as they do in birds.

The windpipe varies much in length, in the number of its rings (which are from 14 to 78), and in their completeness; the cartilages do not generally form a complete circle, being membranous posteriorly, and in the whales the membranous portion is said to lie in trout. The mechanism of the mammalian respiration has been described under Diaphragm, the muscular partition which separates the thoracic and abdominal cavities in this class. - The voice, under the control of the will, is produced by the sage of air from the lungs over certain organs in the larynx or upper portion of the windpipe; in man the larynx is a short and wide tube,' suspended as it were from the hyoid bone, formed of cartilaginous plates, called the thyroid, cricoid, and two arytenoid cartibi-the prominence commonly called "Adanm's apple" is the anterior surface of the thyroid cartilage. The mucous membrane forms two lateral folds from before backward, like the lips of a buttonhole, the vocal cords or ligaments; above these are two other between which and the vocal cords is a cavity on each side, the ventricle of the larynx; the space between these four folds is the glottis which is covered above, during the passage of food or drink, by a tibro-cartilagmous tongue, the epiglottis.

In ordinary respiration the air passes noiselessly; but when the will contracts or otherwise modifies these cords, sound is produced, which in man becomes articulate speech by the action of the pharynx, nasal pas-sages, and parts contained within the mouth. The epiglottis exists in all mammals, but it is sometimes divided at the upper end; in cetaceans, the larynx ascends to the posterior mires and communicates with the blow-hole on the top of the head. The lion's roar depends on the great size of the larynx; the grunt of the hop is produced in cavities communicating with its ventricles; the neigh of the horse by vibration of folds connected with the vocal cords; the bray of the ass by reverberation in a large cavity with small aperture under the thyroid cartilage; in the howling monkeys the hyoid bone is dilated into a bony pouch, and each ventricle opens into a large membranous sac, in which the loud sounds of these animals are produced; in the marsupials the voice is very weak. - The urinary system of mammals consists of secretory organs (the kidneys), and a reservoir for the secreted fluid (the bladder), communicating with the former by the ureters and externally by the urethra.

The kidneys of mammals pre-sent the same external cortical and internal tubular portions as in man, and also the suprarenal capsules, in the lumbar region near the vertebra- and external to the peritoneal sac; they differ somewhat in form, being more or less lobulated, as in the human foetus, in ceta-ceans, seals, otters, bears, the elephant, and ox; the lobules vary from 10 in the otter to 130 in the seals, in cetaceans resembling a hunch of grapes; in all, except the monotre-mata, the ureters open into the bladder; in these into the urethra, as in chelonians. The bladder is generally more loosely connected in other mammals than in man; it is largest in the herbivora, smaller and more muscular in the carnivora and rodents. The chemical composition of the urine is about the same in carnivora as in man. except in the absence of uric arid; in the herbivora it is alkaline, containing hippuric acid and much earthy carbonate. In the stags, below the inner angle of the eyes, there is an opening communicating with a large membranous pouch, from the glands of which is secreted a brownish liquid, flowing down the sides of the face, like tears; many animals have glands on the abdomen, in the groins, or about the genito-anal openings whose secretion is very odorous, as in the musk deer, beaver, civet, and skunk. - The special internal male organs are the testes, which se-crete the sperm, with certain accessorv glands (as the prostate and Cowper's), and seminal receptacles or retir.ula; in the female the germs are formed in the ovaries, whence they 'escape through the Fallopian tubes into the uterrus, and „„.„,.,. when fall.grown externall asthe name mammal imports, they have also external glands for the secretion of milk the mamma' or breasts.

The testes may be permanently external, as in the dog; always abdominal, as in the seal, elephant, and cetaceans; or external during the rutting season, and at other times internal, as in the mole and porcupine. The epididymis is usually largely developed; the seminal vesicles are found in monkeys, bats, rodents, and pachyderms, but are wanting in carnivora, most plantigrades, ruminants, and marsupials; the prostate gland exists in some form in all mammals; the absence of Cowper's glands in most pachyderms, rodents, and carnivora shows that their action is not. essential to reproduction. The human ovaries are two oval, glandular bodies, about an inch long, in the posterior portion of the broad ligaments; each contains about 20 Graafian vesicles, enclosing an ovum. All the internal organs, except the uterus, are much alike in the other mammalia. This last organ, single in the monkeys, is in carnivora, many rodents, pachyderms, ruminants, and cetaceans, generally divided at the base into two horns (cornua), each sometimes having its distinct opening; in marsupials the ovaries are more or less racemose, as in birds.

In most mammals, after the fecundated ovum has descended through the Fallopian tube (in the higher orders about the 12th day), an intimate vascular connection takes place between the sinuses of the parent and the chorion of the foetus, forming the placenta, which continues to supply the young with nutriment until it is capable of an independent existence. The period of utero-gestation, about 270 days in the human mammal, varies in the different families. This group of placental mammals has been called monodelphians to distinguish them from the didelphians, which include the marsupials and monotremata; the former have a more perfect brain, with its hemispheres united by a corpus callosum; the latter bring forth their young in a very imperfect condition, but have the brain destitute of a corpus callosum, the abdominal walls supported in front by two bones arising from the pubis, and an external pouch for the reception of the young. Prof. Jeffries Wyman ("Proceedings of the Boston Society of Natural History," vol. vi., p. 363), from the examination of a large number of foetal pigs, has shown that the above division of mammals into "placentals" and "implacentals " is not well defined; he found that in pigs there is, strictly speaking, no placenta, the maternal and foetal vessels being in relation only by means of very minute diffused villi and slight foldings of the chorion; this condition is intermediate between those of marsupials and ruminants, and shows such a gradual transition in this respect that the former must be brought nearer than has been usually admitted to ordinary mammalia.

Mammary glands exist in both sexes, but serve for purposes of lactation only in the female; the number is generally in relation with the number of the young at a birth; there are 2 in monkeys, the elephant, the goat, and the horse; 4 in the cow, the stag, and the lion; 8 in the cat; 10 in the hog, rabbit, and rat; and 12 or 14 in the agouti. The position also varies; in monkeys and bats they are on the chest, in most carnivora on the chest and abdomen, and in the ruminants far back between the posterior limbs; in marsupials they are concealed within the abdominal pouch. Some mammals are born with the eyes open, and are at once able to run in search of food; many, however, are born with the eyes closed and in a very weak condition; and a few, as the marsupials, leave the uterus in such an imperfect state that they would perish did not the parent place them in her pouch, where they complete their development, each suspended to a teat. In the monotremata (ornithorhynclius, &c), which seem to form the connecting link between the mammals and birds, in addition to the horny bill, cloaca, and bird-like ovaries, there are the form, external covering, skeleton, and milk-secreting glands of the mammals. - As to physical distribution, some mammals dwell entirely in the sea, as the cetaceans and most seals; some of the latter and the sirenoid pachyderms (manatee, &c), live chiefly in fresh water; others, beavers, muskrats, the ornithorhyn-chus, etc, frequent rivers and lakes; but most live upon the land, some on mountains like the chamois and ibex, some on plains like the antelopes and bison, some on trees like the apes, squirrels, and sloths; others sail or fly in the air like the flying lemur and the bats, and others live under ground like the moles.

For these different methods of progression and habits of life, the limbs are variously adapted by modifications of the same few osseous elements, and the study of fossil mammals develops the same order in past geological ages. The study of the geographical distribution of mammals shows that the number of genera and species increases from the poles to the equator, with the exception of the whales and seals, which are most numerous in the polar regions; within the northern arctic circle there are species common to both hemispheres, as the arctic fox, white bear, reindeer, and ermine; in temperate North America the species are different from those of the eastern hemisphere, and in South America even the genera from those of the old world, as those including the peccary, llama, armadillo, ant-eater, sloth, cavy, agouti, vampire bat, marmoset, the howling and prehensile-tailed monkeys; the raccoon and muskrat are exclusively American; the hog, horse, camel, rhinoceros, elephant, lion, tiger, lemurs, and anthropoid apes belong now to the eastern world; the giraffe, hippopotamus, chimpanzee, and most of the antelopes, are African; all the marsupials (except the American opossums) and the monotremata are Australian, while the stags, squirrels, cats, bears, dogs, and bats are absent from this region.

The marsupials, though forming scarcely one fifteenth of the land mammals in the world, constitute three fourths of the mammalian fauna of Australia; exclusive of cetaceans and seals, the rodents form one third of the entire number of species of the world, the bate and carnivora one third, the remaining third being chiefly the monkeys, ruminants, marsupials, and insectivora, according to Van der Hoeven, in Europe, wanting marsupials and monkeys the rodents are one third, hats one sixth, and insectivora about one thirteenth; in North America the species of rodents form perhaps half the entire number of land mammals; the large pachyderms, edentates, and the apes be-long to the warm regions, most of the latter being African; the insectivora are almost peculiar to the northern hemisphere, and the lemurs are most common in the southern. Excepting the whales and hats, mammals do not migrate, but spend the summer and winter in the same locality; the whales pass the summer in the polar regions, and come southward in winter into the lower Atlantic. The phenomena of hibernation or winter-sleep in mammals have been described under the former title. - Mammalogy includes the classification of mammalia.

The mammalia were first separated from other four-footed animals by Aristotle, who called them zootoca or viviparous animals; he divided them into three sections according to their locomotive organs: 1, dipoda. or bipeds; 2, tetrapoda, or quadrupeds; 8, apoda, inipeds or whales. The quadrupeds, including all but man and the cetaceans, he subdivided into two great groups according to the modifications of the organs of touch, in the first of which the ends of the digits are left free for the sense of feeling, the nail being on the upper surface only, and in the second the feet ending in hoofs, corresponding respectively to the yti-guiculata and nngulata of Ray. The unguicu-lateshe divided by the teeth into three families: 1, those with cutting incisors and triturating or flattened molars, like the apes (jnthe-coida) and the bats (dermaptera); 2, those with canine or carnivorous teeth, mreharo-donta or gampsomtcha; 3, those corresponding to the rodents, with the negative character of the absence of canine teeth.

The ungulate or hoofed quadrupeds he divided, according to the organs of motion, into: 1, polywhida or multungulates, like the elephant; 2, disehidae or bisnlcates, including the ruminants (tnery-cizonto) and the hogs; and 8, aschidae or -lidungulates, like the horse. '1 he apodal quadruped's included the cetaceans or cetoda. It thus appears that Aristotle clearly perceived the principles upon which mammals are classified by the best modern naturalists. - This arrangement was not improved upon until John Ray published his Synoptis in 1698 in London, and his improvements relate to the four-looted mammals. In his ungulate quadrupeds he places the solipedous (as the horse), the bisul-cate ruminants (like the ox and rtagj or non-ruminants (as the hog), and the qnadnsulcate (rhinoceros and hippopotamus); in the untrnicnlate the feet ore either bifid (as in the camel), or multifid with digits adhering together cis in the elephant), with distinct depressed digits (as in apes), or compressed (as in carnivore insectivora, rodents, and edentates). - Linnams founded his primary divisions on the locomotive organs, deriving his orders from the modifications of the teeth; in his earlier editions of the Systema Nature, up to the 10th, he called the class quadrupedia, including the cetaceans among fishes; in his 12th edition (1766) he makes seven orders, as follow-: A. Unguiculata: I., primates, with four front cutting teeth, including man, the mon-kevs, and bats (4 genera); II., bruta, with no front teeth in either jaw, including the elephant, walrus, and edentates (6 genera); III., ferae, with front teeth, conical and long canines, including the carnivora, opossum, and insectivora (10 genera); IV., glires, with two front cutting teeth in each jaw, including the rodent- (fi genera). B. Ungulata: V., pecora, with cutting front teeth in the lower jaw, but imne in the upper, including the ruminants (6 genera); VI., belhue, with ohtuse front teeth in both jaws, including the pachyderms generally (4 genera). C. Mutica: VII., cete, with horny or bony teeth, pectoral fins instead of feet, and horizontal flattened tail, including the cetaceans (4 genera). He thus made 40 genera in all.

Linmeus followed Ray in placing the elephant among the unguiculata, an error avoided by Aristotle. In 1798 Cuvier published his Tableau elemcntaire des animaux, in which he laid down the basis of his classification, which was variously modified until the second edition of his Regne animal in 1829 in that work he makes the nine following orders of mammalia: bimana, qua-drumana, carnivora, marsupialia, rodent ia, edenta, pachydermata, ruminantia, and ce-In his first edition the marsupials were ranfted among carnivora, and in the Tableau lentaire there were three grand divisions: I., unguiculata, with the orders bimana, qua-drumana cheiroptera, plantigrada, carnivora, pedimana, rodentia, edentata, and tardi-grada; II.. ungulata, with the orders pachydermata, ruminantia, and solipeda; and III., mntira, with the orders amphibia and cetacea. - The system of Blumenbach, Illiger, and Desernarest differ little from that of Cuvier, ex-cept in the names of the orders and their subdivisions, De Blainvillo (1822) makes in the type osteozoaria, or vertebrates, the sub-type ripara and the classpilifera or mammifera, with the divisions monadelphya and didelphya.

Temminck (1827) makes the 11 orders of man, monkeys, bats, carnivora, marsupials, rodents' edentates, pachyderms, ruminants, cetaceans and monotremata. Fischer, inhis Synopsis Mammahum (1829), makes the nine orders of primates Can and monkeys), cheiroptera (bats),ferae (carnivora). bestirn (insectivora and marsupials), glint (rodents), bruta (edentates and monotremata), bellum (pachyderms and solipeds), pecora (ruminants), and cete (herbivorous and ordinary cetaceans). - McLeay (1821), the founder of the quinary classification, makes five orders of mammals, which may be arranged in a tabular form as follows:

Mammals

1. Ferae.

2. Primates.

3. Glires.

4. Ungulata.

5. Cetacea.

Characters

Carnivorous. Omnivorous. Frugivorous. Frequenting the vicinity of water. Aquatic.

Birds

Raptorea.

Insessores.

Easores.

Grallaiores.

Natatores.

This shows the analogies between mammals and birds, in regard to food and habits, which were afterward modified by Swainson (1835) as follows: I., typical group, quadru-mana, organized for grasping, analogous to insessorial birds; II., sub-typical, ferai, with retractile claws and carnivorous, to the rap-tores; III., aberrant group, including cetacea, eminently aquatic, with very short feet, to natatores; glires, with lengthened and pointed muzzle, to grallatores; and ungulata, with crests on the head, to rasores. - Oken in 1802 divided animals into five classes according to the organs of sense; this view is elaborated in his "Physiophilosophy" (Ray society edition, 1847); of these five classes the fifth and highest is the ophthalmozoa or mammalia, so called because in them the eyes are movable and covered with two perfect lids, the other sense organs having however suffered no degradation; he also calls them thricozoa or pilose animals on account of their hairy covering, and aesthetic or sensorial animals from the completion and combination of all the organs of sense. They belong to bis province of sarcozoa or flesh animals. His divisions are as follows: A. Splanchno-thricozod: order L, rodents; II., edentates and marsupials; III., insectivora and cheiroptera.

B. Sarco-thricozoa: IV., ungulata. C.Aesthesio-thricozoa: V'., unguiculata. Every family of the thricozoa contains five genera, in accordance with the five organs of sense; the human family or genus has also five varieties on the same principle: 1, the skin man, the black African; 2, the tongue man, the brown Australian and Malay; 3, the nose man, the red American; 4, the ear man, the yellow Mongolian; and 5, the eye man, the white European. - Another philosophical system is that of Cams. The mammalia are made the seventh class of his third circle, the cephalozoa. He makes ten orders, as follows: 1, natantia, or herbivorous and carnivorous cetaceans, with evident relations with fishes; 2, reptantia, or monotremata and edentates, related to reptiles; 3, volitantia, bats and flying lemurs, related to birds; 4, mergentia, seals and walrus, a repetition of the first; 5, marsupialia, a repetition of the second; 6, glires or rodents, a repetition of the third; 7, pachydermata, a second repetition of the first; 8, ruminantia, a second repetition of the second, indicated by the fifth, which is half ruminant; 9, ferce, a second repetition of the third; and 10, quadrumana, having relations with man. - The fundamental idea of the classification of Fitzinger (1843) is the same as that of Oken, the class mammalia having five series, according to the development of the organs of sense, and each series three orders, viz.:

Touch. Celacea

1. Balanodea.

2. Delphinodea.

3. Sirenia.

Tastk. Pachydermata

1. Phocina.

2. Obesa.

3. Euminantia.

Smell. Edentata

1. Monotremata.

2. Lipodonta.

3. Tardiarada.

Heading. Unguiculata

1. Glires.

2. Brzcta.

3. Fercc.

Vision. Primates

1. Chiropteri.

2. Ilemipitkeci.

3. Antkropomorphi.

Of the embryological systems of classification may he mentioned those of Von Baer, Van Beneden, and Vogt. Von Baer (1828) proposed the following divisions of this class of his doubly symmetrical or vertebrate type, with osseous skeleton, lungs, an allantois, and an umbilical cord: the cord may disappear early, 1, without connection with the mother (monotremata), or 2, after a short connection with the mother (marmpialia); or the cord may be longer persistent, 1, the yolk sac continuing to grow for a long time, the allantois growing little (rodentia), moderately (insec-tivora), or much (carnivora), or 2, the yolk sac increasing slightly, the allantois growing little and the umbilical cord very long (monkeys and man), continuing to grow for a long time and the placenta in simple masses (ruminants), or growing for a long time and the placenta spreading (pachyderms and cetaceans). According to Vogt (1851), mammals may be arranged in two divisions: I., aplacen-taria, with the orders monotremata and mar-supialia; and II., placentaria, with series 1, composed of the orders cetacea, pachyder-rnata, solidungula, ruminantia, and edentata; series 2, of the orders pinnipedia and carnivora; and series 3, of the orders insectivora, volitantia, glires, quadrumana, and bimana.

Van Beneden (1855), in the class mammalia of his hypocotyledones or hypovitellians (vertebrates), in which the vitellus or yolk enters the body from the ventral side, establishes the ten orders primates, cheiroptera, insectivora, rodentia, carnivora, edentata, proboscidea, un-gulata, sirenoidea, and cetacea. Prof. Baird (in vol. viii. of the "Pacific Railroad Survey," 1857) adopts the following arrangement: A, unguiculata, with the orders: 1, quadrumana; 2, cheiroptera; 3, rapacia; 4, marsupialia; 5, rodentia; and 6, edentata; - B, ungulata, with orders: 7, solidungula; 8, pachyder-mata; and 9, ruminantia; - C, pinnata, with orders: 10, pinnipedia; and 11, cetacea. All of these, except the first, are found in North America; the horse, though not now-existing native, was formerly an inhabitant of this country. Agassiz, in his essay on classification (1857), makes mammals the eighth class of vertebrates, with only the three orders ol | marsupialia, herbirora, and carnivora. - Owen (in the article "Mammalia" in the "Cyclopae-dia of Anatomy and Physiology," 1847; admits in the sub-class of placentalia the ten orders of bimana, quadrumana, cheiroptera, insectivora, carnivora, cetacea. pachydermata, ruminantia, edentata, and rodentia, and in the sub-class implacentalia the orders marsupialia and monotremata; the monkeys by the galeo-pithecus are connected with the cheiroptera, and by the lemurs with the carnivora; the last by otaria are related to cetacea, which in turn have certain affinities with the fishes; the rodents are connected with ruminants by the musk deer; the monotremata lead to reptiles. - Before introducing the more recent classification of mammals by Prof. Owen, according to the cerebral system, the reader should be reminded that until the time Cuvier the principal subdivisions were based. upon the Aristotelian characters derived from the organs of locomotion, the secondary groups being established on the peculiarities of the dental system; Cuvier added others drawn from the osseous and generative systems; De Blainville in 1810 first adopted the division, according to the method of reproduction, into monodelphs, didelphs, and ornithodelpbs, or ordinary mammals, marsupials, and monotremes, retaining for the most part the Liniucan orders.

Classification by the placenta seems to have been first proposed by Sir Everard Home, but, as modified by successive naturalists, leads to many unnatural affinities; placing, for instance, rodents and insectivora with monk' and solipeds, pachyderms, and some ruminants with the carnivorous cetaceans. Prince Bonaparte, in his Systema Vertebratorum (1840), adopts the division of placentalia and implacentalia, subdividing the first into the subclasses of educabilia and ineducabilia, the latter including the orders bruta, cheiroptera, insectivora, and rodentia, with the common character of a single-lobed cerebrum; Prof. Owen regards this as the most important improvement since the establishment of the natural character of the ovo-viviparous or implacental division. In 1845 Isidore Geoffroy Saint-Hi-laire raised the marsupials to the rank of a distinct class, making its subdivisions orders equivalent to those of the placentalia; Owen, however, did not regard them as groups of equal rank and value. In 1849 Prof. Owen, from the consideration of the times of formation and the succession of the teeth, divided mammals into two groups, monophyodonts, or those which generate a single set of teeth the monotremata, bruta. and cetacea), and the diphyodonts, or those which generate two seta of teeth (comprising the great hulk of the class)- at the same time he wished it to bo clearly understood that this dental character is not so associated with other organic characters as to indicate natural or equivalent subclasses.

As early as 1842 he drew attention to the value of the principal modifications of the mammalian brain in regard to their association with concurrent modifications in other sratems of organs; it was not till 1857, howeven, that he felt himself justified in proposing to the Linncean society a fourfold division of this class, based upon the four leading modifications of the cerebral structure. His first and lowest group or sub-class is called lyencephala signifying the loose or disconnected state of the cerebral hemispheres, which leave exposed the olfactory ganglia, the cerebellum, and more or less of the optic lobes, have the surface generally smooth, and the anfractu-osities, when present, few and simple; in this division the absence of the corpus callosum commissure is associated with the marsupial mode of development and the non-development of the placenta; it includes the monogenics and marsupials. The next stage in the development of the brain is where the corpus callosum is present, but the hemispheres leave the olfactory lobes and cerebellum exposed, and are commonly smooth or with few and simple convolutions; these are the UssencepJta-la, or smooth-brained mammals, or rodents, insectivora, bats, and edentates, in many respect-;, in common with the preceding subdivision, resembling birds and reptiles.

The third modification is an increased relative size of the hemispheres, which extend over more or less of the cerebellum and olfactory lobes, and have their surface, except in a few of the lower quadrumana, folded into more or less numerous gyri or convolutions; hence this sub-class is called gyrencepliala; among these are not found marks of affinity with the ovi-para, but the highest mammalian perfection is attained, as shown by the size, strength, activity, sagacity, and docility of many of its members; this sub-class comprises the other orders of mammals, man only excepted. In man the hemispheres overlap the olfactory lobes and cerebellum, extending in advance of the former and further back than the latter; in man only is there what is called a third or posterior lobe, and in him the superficial gray matter attains its highest development through the number and depth of its convolutions; as representing a distinct sub-class of mammalia, and ruling naturally over all the other members of the class, he proposes for man the name of archcncepJiala, signifying that he is master of the earth and of the lower creation.

For details on the characters of the secondary groups and their distribution in time and space, the reader is referred to the original paper in the "Proceedings of the Linnaean Society " of London, vol. ii., pp. 1-37, 1857. His tabular arrangement is as follows:

Class.

Sub-class.

Order.

Family or genus.

Example.

MAMMALIA.

ARCHENCEPHALA......

BIMANA...

Homo...

Man.

QUADRUMANA..........

Catarhina...

Ape

Unguiculata.....

Platyrhina........

. Marmoset.

Gyrescephala.

Strepsirhina......

. Lemur.

Carnivora............

Digitigrada.......

.Dog:.

Plantigrada......

. Bear.

Pinnigrada.......

Seal.

Ungulata........

Omnivora.........

Hog.

Ruminantia......

. Sheep.

Perissodactyla.......

Solidungula.......

. Horse.

Multungula.......

. Tapir.

PROBOSCIDIA...

Elephas............

. Elephant.

Dinotherium......

. Extinct.

TOXODONTIA...

Toxodon...........

"

Nsesodon...........

"

Mutilata.........

SlRENIA................

Manatus..........

Sea cow.

Halicore.....

Dugong.

Cetacea...............

Delphinidas........

Porpoise.

Balmnidae.........

. Whale.

Bruta................

Bradypodidoe,.....

Sloth.

LlSSENCEPHALA...

Dasypodidae...

. Armadillo.

Edentula..........

Ant-eater.

CHEIROPTERIA...

Frugivora..

.Roussette.

Insectivora...

,Bat.

INSECTIVORA...........

Talpidce...........

.Mole.

Erinaceidce.......

. Hedgehog.

Soricidae...........

. Shrew.

RODENTIA.........

Non-clariculata...

. Hare.

Claviculata.......

.Rat.

Marsupialia..........

Rhixophaga........

. Wombat.

Lyencephala..

Poephaga.........

. Kangaroo.

Carpophaga...

. Phalanger.

Entomophaga......

.Opossum.

MONOTREMATA...

Echidna...........

.Echidna.

Ornithorhynchus...

.Duck-bill.

The later classification of Huxley does not differ materially in its orders from that of Owen. - Among the many recent American laborers m the department of mammalian classification may be mentioned Prof. Theodore Gill of Washington, D. C, whose articles on this subject, too long to bo condensed here, will be found in the "Proceedingsof the American Association for the Advancement of Science " for 1870 and 1871. - The fossil mammals must be considered before the student can form an idea of the affinities of the class; these and the orders of existing mammalia will be treated under their respective titles. The mammalian class has existed certainly from the lower oolitic period, and probably from the triassic; during this immense lapse of time genera and species have changed, either that they have been newly created at the several epochs, or, as Darwin and others maintain, have been modified by processes of natural selection and development, many original and intermediate forms having become extinct, and, from the imperfection of the geological record, as yet having afforded no indication of their existence.

None of the mammalian genera of the secondary epoch have been found in the tertiary ones; no genus of the older eocene has been discovered in the newer; very few eocene genera have been found in the miocene, and none in the pliocene; many of the miocene genera are peculiar to that division, and some indistinguishable from existing species begin to appear only in the newer pliocene; while the perissodactyls and omnivorous artiodactyls have been gradually dying out, the true ruminants have been increasing in genera and species. One class of organs seems to govern one order, and another class another order; for example, the teeth, which are so diversified in marsupials and edentates, are remarkable for the constancy of their characters in rodents and insectivora; and as a general rule, the characters from the dental, locomotive, and placental systems are more closely correlated in the gyrencephala than in the two inferior sub-classes.