STRUCTURAL ADAPTATIONS 



381 



however, the keel is relatively shallow. But the pectoral 

 muscles in these cases are found to have obtained increased sup- 

 port from the shoulder girdle. In the Gannet and Cormorant 

 and Boatswain-birds {PluPthon), for example, the keel is pro- 

 duced forwards to an extent unknown among other birds; 

 while the hinder portion of the keel in these birds, and in the 

 larger Petrels, tapers off to finally die out before reaching the 

 end of the sternal plate. When the sternum assumes this form 

 the coracoids are either unusually long, and provide special 

 articular surfaces for the furcula, such as we have already de- 

 scribed, or they are short and broad, a modification which 

 reaches its maximum in the larger Petrels. 



III. 40. — Shoulder-girdle and Sternum of a Cormorant 



Economy of material seems to be striven after in the de- 

 velopment of the sterna of birds which fly but little — in such 

 as the Game-birds, Tinamous and Rails. In the case of the 

 first-named, the body of the sternum is generally reduced 

 greatly by deep notches extending far beyond the middle of 

 the breast plate. In the Tinamous all that remains of the 

 sternal plate is a pair of long slender processes arising on either 

 side of the anterior border of the keel and running backwards 

 to its termination, or beyond. In the Rails, in extreme cases, 

 such as the Water-rails, but little of the body of the sternum 

 remains ; and in no birds of this order is the sternal plate more 

 than moderately large. 



It will have been noticed perhaps that we meet with only a 

 rough approximation, and not a complete uniformity, ol adapta- 



