The breast-bone or sternum (fig. 8, p. 13) of birds shows the same relation to the power of flight that is shown by so many, if not by all, parts of the skeleton. It is relatively a very large bone, and is in all perfectly flying birds furnished in the middle line, below, with a strongly marked keel, the presence of which has given its name to the great group of birds called carinates. The ostrich tribe, from whose sterna the keel is absent, are termed ‘ratite,’ or ‘raftlike.’ The reason for the keel is the attachment of the great pectoral muscle, which is the most important muscle of flight. The sternum often offers useful characters to the systematist. The surface of the bone is sometimes in various degrees fenestrate, or more or less deeply incised, the one condition being an exaggeration of the other, and both the conditions being due to defective ossification. The sternum is attached to the vertebral column by the ribs, which are well developed in all birds, but vary very much in number. A highly characteristic feature of the ribs of birds is a small bony projection of the hinder margin of a certain number of them, called the uncinate processes. These are present in all birds, with the single and remarkable exception of the South American Screamers (_Chauna_, _Palamedea_), a group of birds occupying a rather isolated position, and showing resemblances to a great many different groups. Pelvis. [Illustration: FIG. 13.--PELVIS AND HIND LIMB OF DIVER. _c_, _d_, ilium; 63, ischium; 64, pubis; 65, femur; 66, tibia; 67, fibula; 68, tarso-metatarsus; i.-iv. digits with phalanges numbered.]