segment of the back-bone to the cranium, and by the possession of this pair of condyles the Mammalian skull can always be distinguished from that of Birds and Reptiles. The pieces of which the back-bone are composed are named the _vertebræ_. Those of the neck, the "cervical" vertebræ, are recognised by having no true ribs attached to them, and are, in all Primates, seven in number. Those of the back, or "dorsal" vertebræ, may be distinguished by having articulated to them, on each side, {5}a movable rib, the other end of which is attached to the breast-bone; they follow next to the cervical vertebræ, while to them succeed the "lumbar" vertebræ which carry no complete ribs. The dorsal and lumbar segments vary in number, but together they rarely exceed seventeen. Behind these extend the "sacral" vertebræ--completely ossified together, and lastly, the bones of the tail or "caudal" vertebræ, which may be many or few, according to the length of that appendage. The fore-limb is composed of three segments, the arm, fore-arm, and hand, together with a block by which it is attached to the side of the body. To this block--the blade-bone or _scapula_--is articulated the arm-bone, or _humerus_, which at its elbow-joint hinges with the two bones, the _ulna_ and the _radius_, of the fore-arm, on which in turn the hand is rotated. The hand is made up of three parts, the wrist-bones, or _carpus_, closely united together in two transverse rows with a central bone intervening between them; next the elongated bones of the palm of the hand, or _metacarpus_, one to each finger, and lastly the _phalanges_, or finger-bones, three to each digit, except in the thumb, where there are but two. The hind-limb is formed on exactly the same plan. It has a connecting block--the pelvis; giving suspension to the thigh, with its single bone, the _femur_, to which articulates the leg, with two bones (_tibia_ and _fibula_), and the tripartite foot, composed of _tarsus_, _metatarsus_, and _phalanges_.