RIBS AND STERNUM 363 



crest, projecting downwards from the ventral 

 surface of the sternum, and slightly thickened 

 along its anterior border, which is concave. 



iv. Thejnetostea are a pair of large processes, pro- 

 jecting backwards from the sides of the anterior 

 end of the body. Each divides, close to its 

 origin, into two large flattened processes, the 

 external and middle xiphoid processes, which 

 are expanded at their hinder ends. 



v. The costal surfaces are the lateral borders of the 

 body, in front of the metostea : they are notched 

 for the sternal ends of the ribs. 



vi. The costal processes project forwards and out- 

 wards, in front of the costal surfaces. 



vii. The coracoid grooves are a pair of deep trans- 

 verse grooves at the anterior end of the body, 

 continuous with each other, across the median 

 plane, through a hole in the base of the manu- 

 brium. With them the ventral ends of the 

 coracoid bones articulate. 



C. The Skull. 



The chief characteristics of the bird's skull are : ( its 



great lightness ; (2X the very marked tendency of the oones 

 of the cranium to ruse together, most of the cranial sutures 

 being closed, and the outlines of the bones obliterated, by the 

 end of the first year ; (^ the large size of the orbits, which 

 are separated from each other by a thin vertical interorbital 

 septum ; (jj^the prolongation of the face forwards into a long 

 conical toothless beak. 



The skull consists of the same essential parts as that of 

 the rabbit, from which, however, it differs in the following 

 important points : (1) there is only a single occipital condyle, 

 for articulation with the atlas vertebra ; (2) the base of the 

 skull is strengthened by a large membrane-bone, the basi- 

 temporal ; (3) the orbits are placed almost entirely in front of 

 the cranium, instead of below it, and are separated from 

 each other only by the thin interorbital septum; (4) the 



