Like all other backboned animals, birds have a chain of small bones running along the back, and enclosing a canal in which runs the spinal marrow. In most vertebrates some of the individual vertebræ in the region of the hind limb, the sacral region, are somewhat intimately fused together, forming a more solid structure for the support of the pelvis. In birds the strong coupling of the vertebræ is more marked, and extends to the dorsal region. The mechanical value of this to a flying animal is clear; it is analogous to the tight coupling of an express train, and prevents the back from bending from side to side under the strain produced by the powerful movements of the muscles in flight. The tail vertebræ show some curious modifications in different birds. In the typical carinate bird, the last few vertebræ are fused into a piece which is called the ‘plough-share bone,’ or ‘pygostyle.’ The name of this bone sufficiently indicates its shape; the expanded end of the bone serves as a firm base, upon which rest the strong tail feathers. Now, in the ostrich tribe there are no rectrices comparable in size to those of the flying carinates. Here there is no pygostyle, but the individual vertebræ are small and disconnected. They are, however, few in number, whereas in the Archæopteryx they are numerous, though, oddly enough, not so numerous altogether as are the tail vertebræ of some flying birds. Each individual vertebra in the Archæopteryx supports a pair of rectrices, which are thus arranged in a series, and not in one row. A very distinctive peculiarity of the vertebræ of birds is the saddle-shaped centrum. The centrum of the vertebra is the solid piece which underlies the canal of the spinal cord, the walls of the latter being formed by the neural arches, which unite above to form a neural spine. In other vertebrates the centra are flat (mammals), or procœlous (the concavity being forward), or opisthocœlous (the concavity posterior), or amphicœlous (concave on both sides). This latter form of vertebra is frequently met with in archaic forms belonging to various groups. It occurs, for example, in many fishes. Such reptiles as Hyperodapedon and the Geckos have the same kind of vertebræ. Among birds there is no existing genus or species which is to be thus characterised; but the extinct Ichthyornis had clearly biconcave vertebræ. Shoulder Girdle. [Illustration: FIG. 8.--STERNUM OF SHRIKE. _h_, ribs; 58, furcula; 52, coracoid; 59, anterior end of sternum.]