In front of the neck is the skull, in the Mammals a bony case containing the brain and organs of sense, made up of plates interlocking by their zigzag margins; in the Reptiles and lower vertebrates a more or less open framework. The lower jaw, or mandible, is in adult Mammals the only part of the skull that is separate. Its hinder ends work in cavities on the lower part of the skull, and are held in position by strong ligaments and muscles. The ribs are attached to the dorsal vertebræ, and connect by cartilage at the other end with the sternum or breastbone--really a series of united bones in the middle line of the chest (thorax). The blade-bones (scapula) of the forelimbs are attached to the upper ribs by the flat or concave side; and the hinder limbs are connected strongly to the sacrum by means of the hip-bones which are united below to form the pelvis, to which the thigh-bone is jointed. The Reptiles and Amphibians exhibit some differences in their skeletal structure which will be pointed out later. [Illustration: *Skeleton of the Common Badger.*] In the matter of teeth there is great diversity among the Mammals--even in the small number of British species. With a view to a proper understanding of the teeth in, say, the Rodents and the Ungulates, it is necessary to write a few words respecting tooth-structure. Although in adult Mammals the teeth are so intimately connected with the jaw as to appear outgrowths from it, this is not the case really. They originate in the skin which covers the jaw, and the most effective part of their structure--the enamel--is derived from the epidermis, the outer layer of the skin. The centre of each tooth is filled with pulp, around which is the bone-like dentine with an outer coat of hard, glossy enamel. In the incisors or cutting teeth of the Rodents, while the front of the tooth is protected by a thick plate of hard enamel, the back portion consists only of dentine which wears away whilst the enamel front maintains a chisel-like cutting edge. In the grinding teeth or molars, especially noticeable in the Ungulates, the enamel is thrown into ridges and tubercles, so that the action of these in the upper and lower jaws upon each other is like that of "the upper and the nether millstones" in grinding corn. Four forms of teeth are recognised in the Mammals: the incisors in the front of the jaw, the pointed, round canines or "eye-teeth" next to them, and at the sides the cheek teeth, separated into premolars and molars. In describing the teeth in any species a simple formula is adopted which shows at a glance the number of each kind in one side of each jaw. Taking our own normal dental equipment as an example, it would be expressed in this fashion:-- _i 2/2, c 1/1, pm 2/2, m 3/3 = 32_