22 
ON THE CLASSIFICATION OP BIRDS. 
gins of the upper mandible fold over those of the lower ; 
while in others^ as the fincheSj the two edges meet. The 
h g c il 
line thus formed by the meeting of the mandibles is 
termed the commissure (/) : it is either straight, as in the 
woodpeckers ; festooned, as in the sparrow-hawk ; undu- 
lated, as in some of the tanagers (^ fig. 5 1.) ; arched, as in 
the bullfinch ; or angulated at its base, as in the starlings. 
In many birds, the upper mandible is continued far 
back on the front or forehead, and there dilated, so as 
to resemble a sort of casque or helmet : this is seen in 
the shrike-crows (Barita, Vanga, &c.), in the cassicans, 
and slightly in the finches. In rapacious birds and 
parrots, there is a belt of soft naked skin at the base 
of the upper mandible, in which the nostrils are placed ; 
