28 SPECIAL PARTS OF THE UPPER MANDIBLE. 
house ; it is the upper profile of the bill—the highest middle lengthwise line 
of the bill; it begins where the feathers end on the forehead, and extends 
to the tip of the upper mandible. 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 puffin: but in the vast majority of cases it is con- 
vex, with increasing convexity towards the tip. Sometimes it rises up into 
a thin elevated crest, as well shown in Crotophaga (gen. 126) and in the 
puffins, when the upper mandible is said to be heeled, and the culmen it- 
self 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. 5, a). Ina 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; thus 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 
maxillary tomium, as far backward as it is hard and horny. ‘The most con- 
spicuous feature of the upper mandible in most birds is the 
§ 51. Nasa Fossa (L. fossa, a ditch), or nasal groove (fig. 5, ¢), in 
which the nostrils open. The upper prong of the intermaxillary bone (§ 44) 
is usually separated some ways from the two lateral ones; 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; su/cws or groove when long and narrow ; the 
former is well illustrated in the gallinaceous birds; the Jatter in nearly 
all wading birds and many swimmers. When the prongs are soldered 
throughout, or are very short and close together, there is no (or no evident) 
nasal depression, and the nostrils open flush with the level of the bill. The 
§ 52. Nosrrits (fig. 5, d) vary in position as follows :— they are lateral 
when on the sides of the upper mandible (almost always) ; culminal 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, etc.) ; 
terminal when beyond this (very rare; and probably there are now no birds 
with nostrils at the end of the bill, except the Apteryx). The nostrils are 
pervious, when open, as in nearly all birds; dmpervious, when not visibly 
open, as among cormorants and other birds of the same order; they are 
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