In the Armadillos and their extinct allies alone is there an ossified exoskeleton, or bony covering developed in the skin. In all other mammals the skeleton is completely internal. It may be described as consisting of an axial portion belonging to the head and trunk, and an appendicular portion belonging to the limbs. There 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 centres. 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. _Axial skeleton._—The axial skeleton consists of the skull, the vertebral column (prolonged at the posterior extremity into the tail), the sternum, and the ribs. _Skull._—In the _skull_ of adult mammals, all the bones, except the lower jaw, the auditory ossicles, and the bones of the hyoid arch, are immovably articulated together, their edges being in close contact, and often interlocking by means of fine denticulations projecting from one bone and fitting into corresponding depressions of the other; they are also held together by the investing periosteum, or fibrous membrane, which passes directly from one to the other, and permits no motion, beyond perhaps a slight yielding to external pressure. In old animals there is a great tendency for the different bones to become actually united by the extension of ossification from one to the other, with consequent obliteration of the sutures. The cranium, thus formed of numerous originally independent ossifications, which may retain throughout life more or less of their individuality, or be all fused together, according to the species, the age, or even individual peculiarity, consists of a brain-case, or bony capsule for enclosing and protecting the brain, and a face for the support of the organs of sight, smell, and taste, and of those concerned in seizing and masticating the food. The brain-case articulates directly with the anterior cervical vertebra, by means of a pair of oval eminences, called condyles, placed on each side of the large median foramen which transmits the spinal cord. It consists of a basal axis, continuous serially with the axes or centra of the vertebræ, and of an arch above, roofing over and enclosing the cavity which contains the cephalic portion of the central nervous system (see Fig. 6). The base with its arch is composed of three segments placed one before the other, each of which is comparable to a vertebra with a greatly expanded neural arch. The hinder or occipital segment consists