are also certain bones called splanchnic, being developed within the substance of some of the viscera. Such are the os cordis and os penis found in some mammals. It is characteristic of all the larger bones of the Mammalia that their ossification takes its origin from several distinct centers. One near the middle of the bone, and spreading throughout its greater portion, constitutes the diaphysis, or shaft," in the case of the long bones. Others near the extremities, or in projecting parts, form the epiphyses, which remain distinct during growth, but ultimately coalesce with the rest of the bone. The axial skeleton consists of the skull, the vertebral column (prolonged at the posterior extremity into the tail), the sternum, and the ribs. The appendicular portion of the framework consists, when completely developed, of two pairs of limbs, anterior and posterior. The anterior limb is present and fully developed in all mammals, being composed of a shoulder girdle and three segments belonging to the limb proper, viz., the upper arm or brachium, the fore-arm or antibrachium, and the hand or manus. The posterior limb is constructed upon a plan very similar to that of the anterior extremity. It consists of a pelvic girdle and three segments belonging to the limb proper, viz., the thigh, the leg, and the foot or pes. Digestive System.-The search after the purpose which every modification of structure subserves in the economy is always full of interest, and, if conducted with due caution and sufficient knowledge of all the attendant circumstances, may lead to important generalizations. It must always be borne in mind, however, that adaptation to its special function is not the only cause of the particular form or structure of an organ, but that this form, having in all probability been arrived at by the successive and gradual modification of some other different form from which it is now to a greater or less degree removed, has other factors besides use to be taken into account. In no case is this principle so well seen as in that of the organs of digestion. These may be considered as machines which have to operate upon alimentary substances in very different conditions of mechanical and chemical combination, and to reduce them in every case to the same or precisely similar materials; and we might well imagine that the apparatus required to produce flesh and blood out of coarse fibrous vegetable substances would be different from that which had to produce exactly the same results out of ready-made flesh or blood; and in a very broad sense we find that this is so. If we take a large number of carnivorous animals, belonging to different fundamental types, and a large number of herbivorous animals, and strike a kind of average of each, we shall find that there is, pervading the first group, a general style, if we may use the expression, of the alimentary organs, different from that of the others. There is a specially carnivorous and a specially herbivorous modification of these parts. But, if function were the only element which has guided such modification, it might be inferred that, as one form must be supposed to be best adapted and most perfect in its relation to a particular kind of diet, that form would be found in all the animals consuming that diet. But this is far from being the case. The horse and the ox, for instance, two animals whose food in the natural state is precisely similar, are yet most different as regards the structure of their alimentary canal, and the processes involved in the preparation of that food. Again, the seal and the porpoise, both purely fisheaters, which seize and swallow and digest precisely the same kind of prey in precisely the same manner, have a totally different arrangement of the alimentary canal. If the seal's stomach is adapted in the best conceivable manner for the purpose it has to fulfill, why is not the porpoise's stomach an exact facsimile of it, and vice versa? We can only answer, the seal and porpoise belong to different natural groups of animals, formed on different primitive types, or descended from differently constructed ancestors. On this principle only can we account for the fact that, whereas, owing to the comparatively small variety of the different alimentary substances met with in nature, few modifications would appear necessary in the organs of digestion, there is really endless variety in the parts devoted to this purpose. Circulatory, Absorbent, Respiratory, and Urinary Systems.-The blood of mammals is always red, and during the life of the animal hot, having a nearly uniform temperature, varying within a few degrees on each side of 100° Fahr. The corpuscles are, as usual in vertebrates, of two kinds-(1) colorless, spheroidal, nucleated, and exhibiting amoeboid movements; while (2) the more numerous, on which depends the characteristic hue of the fluid in which they are suspended, are colored, non-nucleated, flattened, slightly biconcave disks, with circular outline in all known species except the camels and llamas, where they have the elliptical form charac teristic of the red corpuscles of nearly all the other vertebrates, though adhering to the mammalian type in absence of nucleus and relatively small size. The heart of mammals consists of four distinct cavities, two auricles and two ventricles. Usually the ventricular portion is externally of conical form, with a simple apex, but in the Sirenia it is broad and flattened, and a deep notch separates the apical portion of each ventricle. A tendency to this form is seen in the Cetacea and the seals. The absorbent or lymphatic system of vessels is very completely developed in the Mammalia. Its ramifications extend through all the soft tissues of the body, and convey a colorless fluid called lymph, containing nucleated corpuscles, and also, during the process of digestion, the chyle, a milky fluid taken up by the lym phatics (here called lacteals) of the small intestine, and pour them into the general vascular system, where they mix with the venous blood. Respiratory Organs.-Mammals breathe occasionally through the mouth, but usually, and in many cases exclusively, through the nostrils or nares. The narial passages have the organ of smell situated in their upper part, and communicate posteriorly with the pharynx, and through the glottis with the "trachea or windpipe, a tube by which the air is conveyed to and from the lungs. The upper end of the trachea is modified into the organ of voice or "larynx," the air passing through which to and from the lungs is made use of to set the edges of the "vocal cords," fibrous bands stretched one on each side of the tube, into vibration. The thoracic cavity of mammals differs from that of the Sauropsida in being completely separated from the abdomen by a muscular partition, the "diaphragm," attached to the vertebral column, the ribs, and the sternum. This is much arched, with the convexity toward the thorax, so that when its fibers contract it is flattened and the cavity of the thorax increased, and when they are relaxed the cavity is diminished. The lungs are suspended freely in the thorax, one on each side of the heart, being attached only by the root, which consists of the bronchus or air-tube, and pulmo nary arteries and veins by which the blood is passed backward and forward between the heart and the lungs. Urinary Organs.-The kidneys of mammals are more compact and definite in form than in other vertebrates, being usually more or less oval, with an indent on the side turned toward the middle line from and into which the vessels and ducts pass. In all mammals except the monotremes the ureters terminate by slit-like valvular openings in the urinary bladder. This receptacle when filled discharges its contents through the single median urethra, which in the male is almost in variably included in the penis, and in the females of some species of rodents, insectivores, and lemurs has a similar relation to the clitoris. Nervous System and Organs of Sense.—The brain of mammals shows a higher condition of organization than that of other vertebrates. The cerebral hemispheres have a greater preponderance compared to other parts, especially to the so-called optic lobes, or corpora quadrigemina, which are completely concealed by them. The twelve pairs of cranial nerves generally recognized in vertebrates are all usually found in mammals, though the olfactory nerves are excessively rudimentary, if not altogether absent, in the toothed whales. The spinal cord, or continuation of the central nervous axis, lies in the canal formed by the neural arches of the vertebræ, and gives off the compound double-rooted nerves of the trunk and the extremities corresponding in number to the vertebræ, through the interspaces between which they pass out to their destination. The sense of touch is situated in the skin generally, but is more acute in certain regions more or less specialized for the purpose by the presence of tactile papillæ, such as portions of the face, especially the lips and end of the snout, and the extremities of the limbs when these are used for other purposes than mere progression, and the under surface of the end of the tail in some monkeys. The organs of the other special senses are confined to the head. Taste is situated in the papille scattered on the dorsal surface of the tongue. The organ of smell is present in all mammals except the toothed whale. smaller size than in other vertebrates, and have a definite flattened oval form, and are inclosed in a more or less firm "tunica albugenia.' The oviduct has a trumpetlike, and usually fimbriated abdominal aperture, and is more or less differentiated into three portions-(1) a contracted upper part, called in man and the higher mammals "fallopian tube;" (2) an expanded part with muscular walls, in which the ovum undergoes the changes by which it is developed into the foetus, called the "uterus; " (3) a canal, the "vagina" separated from the last by a valvular aperture, and terminating in the urino-genital canal, or common urinal and genital passage, which in higher mammals is so short as scarcely to be distinct from the last Mammary glands, which secrete the milk by which the young are nour shed during the first portion of their existence after birth, are present in both sexes in all mammals, though usually only functional in the female. Secondary sexual characters, or modifications of structure peculiar to one sex, but not directly related to the reproductive function, are very general in mammals. They almost always consist of the acquisition or perfection of some character by the male as it attains maturity, which is not found in the female or young of either sex. In a large number of cases these clearly relate to the combats in which the males of many species engage for the possession of the females during the breeding season; others are apparently ornamental, and of many it is still difficult to apprehend the meaning. One of the most certain and fundamental points in the classification of the Mammalia is, that all the animals now composing the class can be grouped primarily in three natural divisions, which, presenting very marked differentiating characters, and having no existing, or yet certainly demonstrated extinct, intermediate or transitional forms, may be considered as subclasses of equal value, taxonomically speaking, though very different in the numbers and importance of the animals at present composing them. These three groups are often called by the names originally proposed for them by De Blainville-(1) Ornithodelphia, (2) Didelphia, (3) MonodelThe organ of sight is quite rudimentary, and even phia-the first being equivalent to the order Monotreconcealed beneath the integument, in some burrowing mata, the second to the Marsupiala, and the third inrodents and insectivores, and is most imperfectly de- cluding all the remaining members of the class. Alveloped in the Platanista, or freshwater dolphin of though actual paleontological proof is wanting, there is the rivers of India. In all other mammals the eyeball | much reason to believe that each of these, as now existhas the structure characteristic of the organ in the ing, are survivors of distinct branches to which the earhigher Vertebrata, consisting of parts through which liest forms of mammals have successively given rise, and the rays of light are admitted, regulated, and concen- for which hypothetical branches Huxley has proposed the trated upon the sensitive expansion of the optic nerve names of Prototheria, Metatheria, and Eutheria, names lining the posterior part of the ball. which, being far less open to objection than those of Blainville, we shall here use as equivalents for the latter. The organ of hearing is inclosed in a bony capsule (periotic) situated in the side of the head, intercalated between the posterior (occipital) and the penultimate (parietal) segment of the skull. Reproductive Organs.—In the male the testes retain nearly their primitive or internal position throughout life in the Monotremata, Sirenia, Cetacea, most Edentata, Hyracoidea, Proboscidea, and seals, but in other orders they either periodically (as in Rodentia, Insectivora, and Chiroptera) or permanently pass out of the abdominal cavity through the inguinal canal, forming a projection beneath the skin of the perineum, or becoming sus pended in a distinct pouch of the integument called scrotum. The penis is almost always completely developed, consisting of two corpora cavernosa attached to the ischial bones, and of a median corpus spongiosum inclosing the urethra, and forming the glans at the distal portion of the organ. In the female, the ovaries retain permanently their original abdominal position, or only descend a short distance into the pelvis. They are of comparatively The characters of the Prototheria can only be deduced from the two existing families, as hitherto no extinct animals which can be referred to other divisions of this remarkable and well-characterized group have been discovered. These two isolated forms, in many respects widely dissimilar, yet having numerous common characters which unite them together and distinguish them from the rest of the Mammalia, are the Ornithorhynchidae and the Echidnide, both restricted in their geographical range to the Australian region of the globe. Taken altogether they represent the lowest type of evolution of the mammalian class, and most of the characters in which they differ from the other two subclasses tend to connect them with the inferior vertebrates, the Sauropsida and Amphibia; for, though the name Ornithodelphia owes its origin to the resemblance of the structure of the female reproductive organs to those of birds, there is nothing especially bird-like about them. Their principal distinctive characters are these:-The brain has a very large anterior commissure, and a very small corpus callosum, agreeing exactly in this respect with the next group. The cerebral hemispheres, in Echidna at least, are well developed and convoluted on the surface. The auditory ossicles present a low grade of development, the malleus being very large, the incus small, and the stapes columelliform. They have no true teeth, though the jaws of Ornithorhyncus are provided with horny productions, which functionally supply their place. The coracoid bone is complete, and articulates with the sternum, and there is a large "interclavicle" or episternum in front of the sternum, and connecting it with the clavicles. There are also "epipubic" bones. The oviducts (not differentiated into uterine and fallopian portions) are completely distinct, and open as in oviparous vertebrates separately into a cloacal chamber, and there is no distinct vagina. The testes of the male are abdominal in position throughout life, and the vasa deferentia open into the cloaca, not into a distinct urethral passage. The penis, attached to the ventral wall of the cloaca, is perforated by a canal in the greater part of its length, but not at the base, which is open as in reptiles and those birds which have such an organ, and brought only temporarily in contact with the termination of the vasa deferentia, so as to form a seminal urethra when required, but never transmits the urinary secretion. This condition is a distinct advance on that of the Sauropsida in the direction of the more complete development of these parts in most of the other Mammalia. The ureters do not open into the bladder, but behind it into the dorsal wall of the genito-urinary passage. The mammary glands have no distinct nipple, but pour out their secretions through numerous apertures in the skin. The early stages of the development of the young are not yet fully known, but they are produced in a very rudimentary condition, and appear never to be nourished by means of an allantoic placenta. The Metatheria or Didelphia are represented at present by numerous species, presenting great diversities of general appearance, structure, and habits, although all united by many essential anatomical and physiological characters, which, taken altogether, give them an intermediate position between the Prototheria and the Eutheria. may first separate a group called Edentata, composed of several very distinct forms, the sloths, anteaters, and armadillos, which under great modifications of characters of limbs and digestive organs, as well as habits of life, have just enough in common to make it probable that they are the very specialized survivors of an ancient group, most of the members of which are extinct, but which the researches of paleontology have not yet revealed to us. The characters of their cerebral, dental, and in many cases of their reproductive organs show an inferior grade of organization to that of the generality of the subclass. The next order, about the limits of which there is no difficulty, is the Sirenia, aquatic vegetable-eating animals, with complete absence of hind limbs, and low cerebral organization-represented in our present state of knowledge by but two existing genera, the dugongs and manatees, and by a few extinct forms, which, though approaching a more generalized mammalian type, show no special characters allying them to any of the other orders. Another equally well-marked and equally isolated, though far more numerously represented and diversified, order, is that of the Cetacea, composed of the various forms of whales, dolphins, and porpoises. In aquatic habits, external fish-like form, and absence of hind limbs they resemble the last, though in all other characters they are as widely removed as are any two orders among the Eutheria. The association by systematists of the Cetacea and Sirenia in one group can only be made either in ignorance of their true structure, or in an avowedly artificial system. All the remaining orders are more nearly allied together, the steps by which they have become modified from one general type being in most cases not difficult to realize. Their dentition especially, however diversified in detail, always responds to the formula already described; and, although the existing forms are broken up into groups in most cases easy of definition, the discoveries already made in paleontology have in great measure filled up the gaps between them. Very isolated among existing Eutheria are the two species of elephant constituting the order called Proboscidea. These, however, are now known to be the survivors of a large series of similar animals, mammoths, mastodons, and Dinotheria, which, as we pass backward, in time gradually assume a more ordinary or genThe Eutheria, Monodelphia, or "placental mam- eralized type; and the interval which was lately supmals" (so called because the fetus is always nourished posed to exist between even these and the rest of the while within the uterus of the mother by means of an class is partially bridged over by the discovery in Ameriallantoic placenta) include at present by far the greater can Eocene and early Miocene formations of the giganproportion of the class. While the survivors of the tic Dinocerata, evidently offshoots of the great group other groups have probably been for a long time in a of hoofed animals, or Ungulata, represented in the stationary condition, these have, as there is already actual fauna by the horses, rhinoceroses, tapirs, swine, good evidence to show throughout all the Tertiary and ruminants. Almost as isolated as the Proboscidea geological age, and by inference for some time before, among existing mammals are the few small species conbeen multiplying in numbers and variations of form, stituting the genus Hyrax, and in their case palæonand attaining higher stages of development and special- tology affords no help at present, and therefore, pendization in various directions. They consequently exhibiting further discoveries, it has been thought advisable in far greater diversity of external or adaptive modification than is met with in either of the other subclasses-some being fitted to live as exclusively in the water as fishes, and others to emulate the aërial flight of birds. To facilitate the study of the different component members of this large group, it is usual to separate them into certain divisions which are called orders." In the main zoologists are now of accord as to the general number and limits of these divisions among the existing forms, but the affinities and relationships of the orders to one another are far from being understood, and there are very many extinct forms already discovered which do not fit at all satisfactorily into any of the orders as commonly defined. Commencing with the most easily distinguished, we most recent systems to give them the honor of an order to themselves, under the name of Hyracoidea. But the number of extinct forms already known allied to the Ungulata, but not coming under the definition of either of the two groups (Artiodactyla and Perrissodactyla) under which all existing species range themselves, is so great that either many new orders must be made for their reception or the definition of the old order Ungulata so far extended as to receive them all, in which case both Proboscidea and Hyracoidea might be included within it. Again the Rodentia, or gnawing animals-rabbits, rats, squirrels, porcupines, beavers, etc.-are, if we look only at the present state of the class, most isolated. No one can doubt what is meant by a rodent animal, or have any difficulty about defining it clearly, at least by its |