40 THE STORY OF THE BIRDS 



into certain horns, knobs, and protuberances, 

 reaching its climax in the Hornbills and Toucans : 

 the Coots, Swans, and Scoters may be also 

 instanced. Reverting to the nostrils we find 

 certain very distinct classes ; those in which the 

 nasal cavities are separated by a septum are 

 termed "impervious," and those in which the 

 septum is incomplete, the nasal cavities com- 

 municating, are termed "pervious." Another 

 type of nostril is found in the Cormorants, for 

 instance, in which it is reduced to a mere slit, 

 and becomes still further emphasised in the 

 Gannets, in which the nasals are absolutely 

 closed, and the birds are reduced to exercising 

 the sense of smell through the mouth. The 

 edges of the mandibles in many birds are more 

 or less deeply indented, notched, or serrated, 

 thus enabling the bird better to seize and devour 

 its prey ; in the Ducks and allied birds these 

 serrations take the form of lamellae used for 

 sifting and securing food. Mention should here 

 be made of that small calcareous protuberance 

 at the tip of the upper bill which is developed 

 in 'the embryonic stage of all birds, and known 

 as the "egg tooth." This "tooth" is used as a 

 file by the chick in freeing itself from the egg- 

 shell, and is cast soon after its purpose has been 



