SEC. in EXTERIOR PARTS OF BIRDS 153 



According to the shape of the bill it may be straight or convex, or 

 concave, or even somewhat m -shaped ; or double-convex, as in the 

 tufted pufBn : but in the great majority of cases it is convex, with 

 increasing convexity towards the tip. Sometimes it rises up into 

 a thin elevated crest, as well shown in the genus Crotophaga, and 

 in the puffins (Fratercula), when the upper mandible is said to be 

 keeled, and the culmen itself to be cultrate ; sometimes it is really a 

 furrow instead of a ridge, as toward the end of a snipe's bill ; but 

 generally it is simply the uppermost line of union of the gently 

 convex and sloping sides of the upper mandible (Fig. 2 6, a). In a 

 great many birds, especially those with depressed bill, as all the 

 ducks, there is really no culmen ; but then the median lengthwise 

 line of the surface of the upper mandible takes the place and name 

 of culmen. The culmen generally stops short about opposite the 

 proper base of the bill ; then the feathers sweep across its end, and 

 downwards across the base of the sides of the upper mandible, 

 usually also obliquely backwards. Variations in both directions 

 from this standard are frequent ; the feathers may run out in a 

 point on the culmen, shortening the latter, or the culmen may run 

 a way up the forehead, parting the feathers ; either in a point, as 

 in the rails and gallinaceous birds, or as a broad plate of horn, as 

 in the coots and gallinules. The lower edge (double) of the upper 

 mandible is the maxUlary tomium, as far backward as it is hard and 

 horny. The most conspicuous feature of the upper mandible in 

 most birds is the 



Nasal Fossa (Lat. fossa, a ditch), or rmsal groove (Fig. 26, c), in 

 which the nostrils open. The upper prong of the intermaxillary 

 bone is usually separated some ways from the two lateral prongs ; 

 the skinny or horny sheath that stretches betwixt them is usually 

 sunken below the general level of the bill, especially in those birds 

 where the prongs are long or widely separated ; this " ditch " is 

 what we are about. It is called fossa when short and wide, with 

 varying depth ; sulcus or groove when long and narrow ; the 

 former is well illustrated in the gallinaceous birds ; the latter in 

 nearly all wading birds and many swimmers. When the inter- 

 maxillary prongs are soldered throughout, or are very short and 

 close together, there is no (or no evident) nasal depression, the 

 nostrils then opening flush with the level of the bill. The 



Nostrils (Fig. 26, d), two in number, vary in position as follows : 

 — they are lateral, when on the sides of the upper mandible 

 (almost always) ; mlminal, when together on the ridge (rare) ; 

 superior or inferior when evidently above or below midway betwixt 

 culmen and tomia ; they are basal, when at the base of the upper 

 mandible ; sub-basal when near it (usual) ; median when at or near 

 the middle of the upper mandible (frequent, as in cranes, geese, 



