5/6 MANUAL OF ZOOLOGY. 



amalgamated in the adult so as to form a single piece, and the 

 sutures even are obliterated, the lower and upper jaws alone 

 remaining movable. The occipital bone carries a 'single occi- 

 pital condyle only, and this is hemispherical or nearly globular 

 in shape. The nasal bones are short, so that the nostrils (ex- 

 cept in Apteryx) are placed far backwards. The " beak " (fig. 

 321), which forms such a conspicuous feature in all birds, con- 



Fig. 321. Skull of Spur-winged Goose (P lectropterus Gambensis). 



sists of an upper and lower half, or a "superior" and "in- 

 ferior mandible." The upper mandible is composed princi- 

 pally of the greatly elongated and coalescent intermaxillary 

 bones, which give off long " frontal processes," and are flanked 

 by the comparatively small superior maxillae. The inferior 

 mandible is primitively composed of twelve pieces, six on each 

 side ; but in the adult these are all indistinguishably amalga- 

 mated with one another, and the lower jaw forms a single 

 piece. As in the Reptiles, the lower jaw articulates with the 

 skull, not directly, but through the intervention of a distinct 

 bone the quadrate bone which always remains permanently 

 movable, and is never anchylosed with the skull. In no living 

 bird are teeth ever developed in either jaw, but both mandibles 

 are encased in horn, forming the beak, and the margins of the 

 bill are sometimes serrated. The quadrate bone is movable, 

 and has articulated to it in fro'nt the slender rod-like "jugal" 

 bone or " quadrato-jugal," which is, in turn, immovably united 

 with the slender maxilla on each side. When the mandible is 

 depressed, the quadrate bone is thrust forward, and the rod 

 formed by the coalescent jugal and maxilla on each side ele- 

 vates the upper half of the beak, which is usually articulated in 

 a more or less movable manner with the front of the skull. 

 The Parrots possess this movable articulation of the upper jaw 

 with the skull in its greatest perfection ; but it exists in a less 

 complete form in most birds. The maxillae of birds, as before 

 remarked, are comparatively slender bones ; but they send in- 



