The hind-limb is connected with the verte bral column through the pelvic girdle, which is united with the sacrum. The bones of the hind-limb, which are homologous to those of the fore-limb, are the femur or upper leg-bone, the tibia and the fibula or lower leg-bone, the tarsal or ankle bones and the metatarsals and phalanges constituting the hind-foot. The peculiarities of the bones of the fore-feet in ungulates, already mentioned, are found also, with only slight modifications, in the hind-feet.
The terminal phalanges of both fore and hind feet are compressed and pointed in beasts of prey and such as climb or dig, forming claws, which are covered with horny sheaths. In large running mammals, the terminal pha lapges are more or less broad and flat and like wise covered with horny sheaths, forming hoofs or nails. Certain bones not connected with the skeleton, such as the or penis, os cordis, etc., are developed in the viscera of various mam mals.
Teeth.— In mammals, unlike the lower ver tebrates, teeth are not produced indefinitely, but in fixed number. At most two visible sets are developed. Traces are found, however, of at least one pre-milk and one post-permanent dentition. The first, called the milk dentition, appears during infancy, being replaced by the second set, called the permanent dentition, as maturity is reached. The milk teeth are less numerous and usually smaller than those of the permanent set. Teeth occur only in the pre maxillary and maxillary bones and the man dibles or lower jaw. The upper teeth are divided into incisors, which are implanted in the premaxillae; canines, which are almost invari ably simple and stand immediately behind the suture between the premaxilla and maxillae; and premolas' and molars, which occupy the edges of the maxilla. The premolars have "milk) predecessors, while the molars have not. The nomenclature of the teeth of the lower jaw is the same as for those of the upper jaw, their character being determined by their rela tion to the latter and by their form and mode of development. While the greatest diversity exists as to the form of the individual teeth, and the development of the dentition as a whole, there are rarely more than 44 in all. The ex ceptions are among the marsupials, where the number rises to 54 in the marsupial anteater, Myrmecobius, and to 64 in a fossil form Am. phitherium; also among the cetacea, one species of which has as many as 246 simple teeth. The true anteaters, Myrmecophagidce, and the spiny anteaters of Australia, Echidna, are with out teeth at any time, but many mammals which do not possess them when adult have rudi mentary teeth in the foetal stages. Such is the case with the whalebone whales, and with the platypus, Ornithorhynchus. The rudimen tary teeth in these disappear early and are re placed by whalebone in the case of the whales, and by horny plates resembling teeth in the platypus.
Teeth consists of two portions, the root and the crown. When most complex they contain three structural elements, the enamel, the den tine and the cement. The enamel is hardest and is restricted to the crown, while the den tine • makes up the mass of the tooth, and the cement usually surrounds the root, or fills spaces between the enamel-folds of the crown. The
enamel develops from •he epithelial tissue of the jaws, the dentine from the deeper-lying areolar tissue and the cement from the walls of the tooth-capsule. Some teeth, such as the incisors of rodents, the tusks of the elephant, etc., grow continuously during life; other com plete their growth early. Especially remarkable forms of teeth are the tusks of elephants, which are incisors, and the tusks of the narwhal, the boar and the babirussa, which are canines. When the crowns of the teeth greatly exceed the roots in height, as in the horse, the teeth are said to be hypsidont or hypselodont; when the reverse is the case, the teeth are called brachydont. Teeth having the crown in the form of tubercles, as in the hog. are called bunodont; those with transverse ridges, as in the ox, many rodents, etc.. are called lophodont.
Alimentary Canal.--The mouth, or entrance to the alimentary canal, contains the tongue, which in the majority of mammals is so at tached below that it can be protruded but a short distance, but is often sufficiently free to be used in grasping food and turning it about in the mouth during the process of mastication. In those mammals which feed upon ants and termites, such as the anteaters, pangolins, etc., and also in certain fruit-eating bats, the tongue is very long and slender and can be extended far beyond the mouth. On the posterior sur face of the tongue are -the organs of taste, and the upper surface is often roughened by horny papillae. A number of large glands, called salivary glands, open into the mouth. Their function is to moisten the food and ini tiate the process of digestion. The glands most constantly present are the parotid, situated at the base of the ear, and opening inside the cheek, and the submaxillary, situated near the angle of the lower jaw, and opening under the apex of the tongue. At the back of the mouth is the entrance to the cesoohagus or gullet, usually a simple tube, leading to stomach. The stomach is an oblong, curved sac, usually enlarged at tht cardiac end where the cesophagtas is attached, and smaller at the lower, or pyloric, end, where it joins the intes tines, It is usually simple, but in the ruminants and the cetaceans consists of several cham bers. The intestities join the stomach at the pyloric end. They are, usually of great length, and divided into two distinct sections. The portiop ,nearest the stomach, called the small intestine, is joined below by one of larger diameter called the large intestine. The upper end of the latter is frequently dilated, forming pouch called the mown, which in -herbivorous mammals, and notably in rodents and many ungulates, is greatly enlarged or elongated. In man, the higher apes and the marsupial wombat it terminates in a narrow prolongation called the vermiform appendix. Different sections of the small intestine have received the names duodenum, jejunum and ileum; and of the large intestine, colon and rectum. The inferior orifice of the, intestines is the anus or vein. Generally speaking, the intestines and catcum are shortest in carnivorous mammals and long est in such as are vegetable feeders, but the carnivorous whales and seals, which have long intestines, form a conspicuous exception.