224 FLAT BILLS. 



covered. As they are unable in all cases to see 

 their food, they have the bill with a covering more 

 or less sentient. They have the mandibles broad 

 and flat, though often with an enlargement at the 

 base of the upper one. 



The gradation is from the geese, which feed fully 

 more on the humid meadows than in the water, to 

 the swans and swimming ducks, which never plunge 

 the body, to the diving ducks which go to the bottom 

 in considerable depths ; and thence it passes to the 

 habitual divers, which capture their food in the water, 

 not at the bottom, and thus have the bill of a different 

 form. As this gradation proceeds, the birds become 

 more and more animal in their feeding. 



Geese, which feed chiefly upon vegetable sub- 

 stances, have the bill elevated at the base, narrowed 

 and rounded at the tip, comparatively short and 

 stout, furnished with a nail at the point of the upper 

 mandible of harder texture than the rest of the bill, 

 and often of a different colour. The sides of the bill, 

 which come in contact for a considerable breadth, 

 are fringed with cartilaginous protuberances resem- 

 bling teeth. This form of bill cuts grass something 

 in the same way as the ruminant mammalia, which 

 have the anterior part of the one jaw with cartilagi- 

 nous ridges in place of teeth. 



Swans, which are also chiefly vegetable in their 

 feeding, but which feed more upon the roots of plants 

 under water, have the upper mandible enlarged at 

 the base, but the bill is larger than in the geese, of 

 equal breadth throughout its length, and not so firm 

 and robust 



Swimming ducks, which are omnivorous, but prefer 

 animal substances picked up on the land or dabbled 

 for in the water, vary a good deal in their habits, and 



