728 



PHYLUM CHORDATA : CLASS AVES — BIRDS 



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. 



There is a well-developed sternum^ generally with a keel., 

 with a separate centre of ossification, to which the pectoral 



Fig. 429. — Anterior aspect of a dorsal vertebra of an ostrich. 



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

 of the centrum ; A.Z., anterior articular process or zygapophysis. 



muscles are in part attached. The strong coracoids reach 

 and articulate with the sternum. In flying birds the clavicles 

 are usually well developed, and connected by an interclavicle, 

 which may be connected with the apex of the sternum. The 

 fore-limb has not more than three digits (I., II., and III.), the 

 three metacarpals are fused {except in Archaeopteryx), and 

 there are only two separate carpals, the others fusing zvith the 

 tnetacarpals, and thus forming a carpo-metacarpus. The 

 thumb is often clawed, the second digit rarely. 



