58 
ON THE CLASSIFICATION OF BIRDS. 
of birds ; or 2. behind its basal margin, as in the 
toucans ; or, 3. towards the very centre of the bill 
— intermediate between the front of the head and the 
tip of tile under mandible, — as in the genus Musophaga 
{fig. 33.). The first of these forms is common to most 
birds, but there is yet great variation in the shape of 
the aperture ; sometimes it is round, sometimes oval, 
and sometimes linear {fig. 2.5. a). Round apertures 
are most prevalent among the falcons, the owls, the 
cuckoos, and the parrots, where the margins are 
generally tumid or swelled. In the fern-owls {Ca- 
primulgida:), and in the cuckoo {fig. 24*.), the aper- 
ture is not only round, but slightly tubular, sometWng 
analogous to many of the bats. In the other fissirostral 
birds, the opening is more oval, and is placed either at 
the extreme termination of 
a membranaceous skin, which 
fills up the depression of the 
part where the nostrils are 
situated, or a little on one 
side; between these, and such 
as are of a lengthened oval, 
there are innumerable grades 
of variation. Linear nostrils 
have also their own peculiar modifications. The grallatorial 
birds show us this structure in its highest state of de- 
velopment On looking to the bill oT the common 
plover, we observe that there is a groove running in the 
middle of the upper mandible of the bill for nearly two 
thirds its length ; and that 
the aperture of the nostrils 
assumes the form of a mere 
slit at the base {fig. 25. a). 
A similar structure is seen 
in the different snipes, cur- 
lews, and sandpipers. In the 
herons, this slit is without the groove ; while in the 
humming birds {fig. 25. h) it is often so completely hid 
