60 
ON THE CLASSIFICATION OP BIRDS. 
Ptilonorhynchus (fig. 17.), as its name implies, has the 
base of the bill entirely covered with small close-set 
feathers instead of bristles ; but unfortunately we know 
nothing of the economy which renders this structure 
necessary. The second mode in which the nostrils are 
developed, is almost peculiar to the toucans, and has 
already been adverted to. The third structure is 
equally rare, and most developed in that magnificent bird 
the Mmophaga violacea (fig. 33.), or violet plantain- 
eater ; we know not, however, the manner in which 
this peculiar structure harmonises with the functions 
of hfe. 
(57.) We now come to the Bill, or rather to the 
jaws of birds, the two mamUbles of which are merely 
the jaws of a vcrtebrated animal lengthened out into a 
rostrum or beak, analogous to that form which is seen 
in crocodiles, garfish, and even in long-snouted quad- 
rupeds. In birds, however, these parts are altogether 
naked, of a horny substance outside, and are unprovided 
with teeth, properly so called. The upper of these pieces 
forms the superior, and the under the inferior, mandible; 
while both collectively constitute tlie bill. It is by 
this organ that the food is seized, or laid hold of, by 
ordinary birds, or torn in pieces by the rapacious tribes. 
The absence of teeth, in such birds as seize their food 
by the bill, or in which this member has superior 
powers of prehension, is supplied by a notch, which forms 
a tooth-like projection close to the point of one or both 
mandibles : but in some of the rapacious birds, as the 
sparrow hawk, this process is placed rather more to- 
wards the middle of the bill, and is rather in the shape 
of a festoon (fig. 27. a) ; whereas, in the typical falcons 
(b), the notch is so deep as to produce a sharp tooth-like 
angle. The owls, on the other hand, have neither the 
one nor the other of these projections on the upper 
mandible (c) ; bnt stiU the tip is as much curved as in 
the two former, and the under mandible is toothed. It 
is almost unnecessary to say how greatly this structure 
facilitates the operation of tearing and dividing the 
