GENERAL CHARACTERS OF RIRDS. 



649 



the pterygoids usually articulate freely with the basisphenoid, 

 the lachrymals may remain free, and there may be a joint in 

 the beak at the end of the premaxilla. There is but one 

 condyle. A membrane bone called the basitemporal covers 

 the basisphenoid. There is an interorbital septum formed 

 from presphenoid and mesethmoid. The otic bones fuse with 

 adjacent bones and with one another about the same time. 

 In modern birds there are no teeth, but the jaws are covered 



KIG. 354A. Anterior aspect of a dorsal vertebra of an ostrich. 



N.SP., Neural spine ; T. P., transverse process; R., facet for a rib ; C., the front end 

 of the centrum ; A.Z., anterior articular processes or zygapophyses. 



by horny sheaths. The premaxillcz are large, and form most 

 of the beak. There is a complete infra-temporal arcade 

 formed by a delicate jugal and quadrato-jugal reaching back 

 to the quadrate. The supra-temporal arcade is usually 

 incomplete, but in some cases a process of the squamosal joins 

 a postorbital process of the frontal. The lower jaw consists 

 on each side of five membrane bones and a cartilage bone 

 the articular which works on the quadrate. Many of the 

 skull bones have a spongy texture, due to cavities filled with 

 air from the nasal and Eustachian tubes. 



