THE SKULL. 409 



bone, projecting from the anterior end and 

 ventral surface of the body. 



iii. The carina, or keel, is a very prominent trian- 

 gular crest projecting downwards from the 

 ventral surface of the sternum, and slightly 

 thickened along its anterior border, which is 

 concave. 



iv. The metostea are a pair of large processes, 

 projecting backwards from the sides of the 

 anterior end of the body of the sternum. 

 Each divides, close to its origin, into two 

 large flattened processes, the external and 

 middle xiphoid processes, which are ex- 

 panded 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 on the anterior end of the body 

 of the sternum, continuous with each other, 

 across the median plane, through a hole in 

 the base of the manubrium. With them the 

 ventral ends of the coracoid bones articulate. 



C. The Skull. 



The chief characteristics of the bird's skull are : (1) Its 

 great lightness ; (2) the very extensive fusion of the bones of 

 the cranium with each other, most of the cranial sutures 

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

 the end of the first year ; (3) the large size of the orbits, 

 which are separated from each other by a thin vertical in- 

 ter-orbital septum ; (4) 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 



