RIBS AND STERNUM. Well-developed ribs are attached to the posterior cervical vertebrae as well as to the thoracic vertebrae. The ribs generally have uncinate processes and separate capitula and tubercula, but uncinate processes are absent in _Chauna Palamedea_ and apparently in _Archaeopteryx_. The sternum (fig. 63) is greatly developed in all birds. In the embryo[107] it is seen to be derived from the union of right and left plates of cartilage, formed by the fusion of the ventral ends of the ribs. In the Ratitae and a few Carinatae, such as _Stringops_, it is flat, but in the great majority of birds it is keeled, though the development of the keel varies greatly. It is large in the flightless Penguins, which use their wings for swimming. Traces of an interclavicle may occur in the embryo. PECTORAL GIRDLE. [Illustration FIG. 63. SHOULDER-GIRDLE AND STERNUM OF A. BLACK VULTURE (_Vultur cinereus_) × 1/3. B. PEACOCK (_Pavo cristatus_) × 3/8. C. PELICAN (_Pelicanus conspicillatus_) × 1/3. (All Camb. Mus.) 1. carina of the sternum. 2. coracoid. 3. scapula. 4. clavicle. 5. costal process. 6. surfaces for articulation with the sternal ribs. 7. xiphoid processes. 8. fontanelle.] The pectoral girdle is also strongly developed in all Carinatae, but is much reduced in Ratitae. In some Moas the sternum has no facet for the articulation of the coracoid, and the pectoral girdle appears to have been entirely absent; it is extremely small also in _Apteryx_. Clavicles are generally well developed in the Carinatae, and small ones are found also in _Hesperornis_, and in Emeus and Cassowaries. In the other living Ratitae and in _Stringops_ they are absent. In some Parrots, Owls and Toucans they do not meet one another ventrally. Clavicles are especially stout in some of the birds of prey. They do not generally touch the sternum, but sometimes, as in the Pelican (fig. 63, C), Adjutant and Frigate bird, they are fused with it. In all Ratitae the scapula and coracoid lie almost in the same straight line with one another, in the Carinatae they are nearly at right angles to one another. ANTERIOR LIMB. In the wing of nearly all birds the ulna is thicker than the radius, but in _Archaeopteryx_ the two bones are equal in size. In the wing of _Archaeopteryx_ there are three long digits with distinct metacarpals. In all other birds the digits are modified, the metacarpals being commonly fused and the phalanges reduced in number. In _Palamedea_ and some other birds the metacarpus bears a bony outgrowth, which when sheathed in horn forms a spur.