THE BILL 17 



the greater range. The ridge along the upper side of the 

 upper mandible is the culmen, that along the lower side of the 

 lower is the gonys. The gonys extends from the tip of the bill 

 to where the central ridge forks ; these two ridges at the base 

 are called the rhami (singular rhamus). The angle of the gonys 

 is between the gonys and the rhami. 



The openings in the upper mandibles are the nostrils. These 

 openings are frequently found in grooved portions of the bill ; 

 in such cases the groove is called the nasal fossa (plural /ossce). 

 The gape is the whole opening of the mouth. Though rictus is 

 sometimes used to mean the same thing, it is usually and more 

 properly restricted to the back corner of the mouth as ex- 

 plained below. The term commissure is used to indicate the 

 edges of the mouth when closed, and the commissural point or 

 angle is the back angle of the mouth. The word tomia is used 

 to indicate the cutting edges of the mandibles. The rictus 

 proper extends from the basal end of the tomia to the corner 

 of the mouth. 



The covering of the bill. — The sheath-like covering of the 

 mandibles is usually hard and horny as in the sparrows, but in 

 many groups of birds it becomes, in part or as a whole, soft 

 and skin-like, and is furnished with nerves of feeling. Most 

 water birds, especially, have soft, leathery, or skin-like and very 

 sensitive coverings to the bills, for feeling the food in the mud 

 at the bottom of the water. A duck has a hard, so-called 

 nail at the tip of the upper mandible.^ A pigeon has a bill, 

 soft at base and hard at tip, and a soft, swollen membrane at 

 the top base of the upper mandible, roofing the nostrils.^ 

 Eagles, hawks, and parrots have a peculiar covering over the 

 base of the upper mandible extending beyond the nostrils.^ 

 This covering is so peculiar that it is given a special name, the 

 cere,* because it frequently has a waxy appearance. In the 



2 

 apgar's birds. — 2 



