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THE NOSTRILS AND NASAL FOSSA4—THE GAPE. 29 
perforate when there is no septum (partition) between them, so that you can 
see 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, ob- 
long, 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 formed 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 cither 
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, ete. A very curious case of this is seen in the 
European wryneck (Jinx 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 antiw) 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. 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 rug, embracing the whole base of the 
upper mandible; as in nuthatches, titmice, redpoll linnets, snow buntings 
and other northern Fringillide. Bristles or feathers thus growing forwards 
are called retrorse (Li. 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. 
§ 53. Tue Garr. It only remains to consider what results from the re- 
lations 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 (L. 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 
