CHARACTERS OF BIRDS. 383 



share-shaped. In the extraordinary Mesozoic bird, the Archa- 

 opteryx macrura, there is no ploughshare-bone, and the tail 

 consists of twenty separate vertebrae, all distinct from one 

 another, and each carrying a pair of quill -feathers, one on 

 each side. As the vertebrae of the ploughshare-bone are 

 distinct from one another in the embryos of existing birds, the 

 tail of the Arcfueopteryx is to be regarded as a case of the per- 

 manent retention in the adult of an embryonic character. In 

 the increased number of caudal vertebrae, however, and in some 

 other characters, the tail of the Archaopteryx makes a decided 

 approach to the true Reptiles. 



The various bones which compose the skull of Birds are 

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

 sutures even are obliterated, the lower jaw alone remaining 

 movable. The occipital bone carries a single occipital condyle 

 only, and this is hemispherical or nearly globular in shape. The 

 "beak" (fig. 328), which forms such a conspicuous feature in all 



Fig. 328. Skull of Spur-winged Goose (Plectropterus Gambetisis). 



birds, consists of an upper and lower half, or a " superior " and 

 " inferior mandible." The upper mandible is composed almost 

 entirely of the greatly-elongated intermaxillary bones, 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 amal- 

 gamated 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 or tympanic bone which always re- 

 mains permanently movable, and is never anchylosed with the 

 skull. In no 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 thoracic cavity is bounded by the dorsal vertebrae, which 



