EXTERNAL PARTS OF BIRDS.—THE BILL. 105 
birds ; impervious, when not visibly open, as among cormorants and other birds of the same 
order ; they are perforate when there is no septum (partition) between them, so that you can 
look through them from one side of the bill to the other, as in the turkey-buzzard, crane, ete. ; 
imperforate when partitioned off from each other, as in most birds ; but different ornithologists 
use these terms interchangeably. The principal shapes of the nostrils may be thus exhibited : 
—a line, linear nostrils; a line variously enlarged at either end, clavate, club-shaped, oblong, 
ovate nostrils ; a line, enlarged in the middle, oval or elliptic nostrils; this passing insensibly 
into the circle, rownd or circular nostrils ; and the various kinds of more or less linear nostrils 
may be either longitudinal, as in most birds, or oblique, as in a few; almost never directly 
transverse (up and down). Rounded nostrils may have a raised border or rim ; when this is 
prolonged they are called tubular, as in some of the goatsucker family, and in all the petrels. 
Usually, the nostrils are defined entirely by the substance surrounding them; thus, of cere, in a 
hawk; of softish skin, in a pigeon, plover or snipe; or of horn, in most birds ; but often their 
contour is partly formed by a special development, somewhat distinct either in form or texture, 
and this is called the nasal scale. Generally, it forms a sort of overhanging arch or portico, 
as well shown in all the gallinaceous birds, among the wrens, etc. A very curious case of 
this is seen in the European wryneck (Lynx torquilla), where the scale forms the floor instead 
of the roof of the nostrils. The nostrils also vary in being feathered or naked ; the nasal fossa 
being a place where the frontal feathers are apt to run out in points (called anti@), embracing 
the root of the culmen. This extension may completely fill and hide the fossa, as in many 
grouse and ptarmigan; but it oftener runs for a varying distance toward, or above and beyond, 
the nostrils ; sometimes similarly below them, as in a chimney-swift ; and the nostrils may be 
densely feathered when there is no evident fossa, as in an auk. When thus truly feathered in 
varying degree, they are still open to view; another condition is, their being covered over 
and hidden by modified feathers not growing on the bill itself, but on the forehead. These 
are usually bristle-like (setaceous), and form two tufts, close-pressed and directed forwards, as 
is perfectly shown in a crow; or, the feathers may be less modified in texture, and form either 
two tufts, one over each nostril, or a single ruff, embracing the whole base of the upper 
mandible ; as in nuthatches, titmice, red-poll linnets, snow buntings and many other northern 
Fringillide. Bristles or feathers thus growing forwards are called retrorse (Lat. retrorsum, 
backward ; here used in the sense of in an opposite direction from the lay of the general 
plumage; but they should properly be called antrorse, 7. e., forward). The nostrils, whether 
culminal or lateral, are, like the eyes and ears, always two in number, though they may be 
united in one tube, as in the petrels. 
The Gape. —It only remains to consider what results from the relations of the two 
mandibles to each other. When the bill is opened, there is a cleft or fissure between them ; 
this is the gape or rictus (Lat. rictus, mouth in the act of grinning). But while thus really 
meaning the open space between the mandibles, it is generally used to signify the line of their 
closure. Commissure (Lat. committere, to put or join together) means the point where the 
gape ends behind, that is, the angle of “the mouth, angulus oris, where the apposed edges of the 
mandibles join each other; but, as in the last case, it is loosely applied to the whole line of 
closure, from true commissure to tip of the bill. So we say, ‘‘ commissure straight,” or ‘* com- 
missure curved ; ” also, ‘‘ commissural edge” of either mandible (equivalent to ‘‘tomial edge”) 
in distinction from culmen or gonys. But it would be well to have more precision in this 
matter. Let, then, tomia (fig. 26, 7) be the true cutting edges of either maudible from tip to 
opposite base of bill proper; rictus (fig. 26, g) be their edges thence to the POINT commis- 
sure (fig. 26, h) where they join when the bill is open; the LINE commissure (fig. 26, f) to 
include both when the bill is closed. The gape is straight, when rictus and tomia are both 
Straight and lie in the same line ; curved, sinuate, when they lie in the same curved or waved 
