740 



PHYLUM CHORDATA : CLASS AVES — BIRDS 



are usually cartilaginous. On the posterior surface of each 

 of the first four thoracic ribs there is an uncinate process, 

 absent only in the S. American screamers (Palamedeae). 



The skull has a rounded cranial cavity, large orbits, and 

 a narrow beak, which is mostly composed of the pre- 

 maxillae. All the bones are fixed except the quadrate, 



lower jaw, columella, and hyoid. 

 The surface is polished ; the 

 sutures are obliterated very early 

 in life. 



The back part of the skull is 

 formed by the basioccipital, the 

 two exoccipitals, and the supra- 

 occipital, surrounding the fora- 

 men magnum. The basioccipital 

 forms most of the single condyle. 

 The roof of the skull is formed 

 from the paired parietals, frontals, 

 and nasals, the last being small 

 and in part superseded by the 

 upward extension of the pre- 

 maxillae. 



The line of the upper jaw 

 consists of premaxilla, small 

 maxilla, jugal, and quadrato- 

 jugal, the last abutting on the 

 movable quadrate. 



Of the membrane bones on the 

 side of the skull, the lachrymal 

 in front of the orbit, and the 

 squamosal between the quadrate 

 and the parietal, are the most 

 important. 



On the roof of the mouth, the 

 basisphenoid, which lies just in front of the basioccipital, 

 is covered over by a membrane bone — the basitemporal. 

 In front of this is a sharp " basisphenoid rostrum " or 

 parasphenoid, also a membrane bone. Articulating with 

 the quadrate and with the rostrum are the pterygoids ; in 

 front of these lie the palatines. The vomer is vestigial. 

 The bony front of the palate is formed from inward 



Fig. 440. — The vertebral 

 or upper part of the rib 

 of a bird. 



C, Capitulum, articulating Oj^ the 

 Centrum of a vertebra ; T., 

 tubercle, articulating on the 

 transverse process of a vertebra ; 

 U.P., an uncinate process, over- 

 lapping the next rib. 



