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 perfordfe 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, etc. ; 

 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, round 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 gaEinaceous 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 antice), embracing 

 the root of the culmen. This extension may completely till 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 stiU 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-poU linnets, snow buntings and many other northern 

 Fri/ngilUdce. Bristles or feathers thus growing forwards are called retrorse (Lat. retrorsum, 

 backward ; here used in the sense of in an opposite di/recUon from the lay of the general 

 plumage; but they 'should properly be called amtrorse, i. 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 i-eally 

 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 amgle 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 biU. 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, j) be the true cutting edges of either mandible from tip to 

 opposite base of biU. proper; rictus (fig. 26, g) be their edges thence to the point commis- 

 sure (fig. 26, h) where they joiji when the bill is open ; the line commissure (fig. 26, /) 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 ; cwrved, sinuate, when they lie in the same curved or waved 



