54 ON THE CLASSIFICATION OF BIRDS. 
tonguCj of less importance than those leading forms we 
have now described ; but as our space wiU not allow of 
extending our remarks upon tliis organ, we shall at once 
proceed to notice others. 
(54.) The Mouth, or rictus, is of greater or less ex- 
pansion, according to the description of food upon which 
the bird subsists, or the method of taking it, although 
in the generality of species the mouth is not of great 
extent ; its angles or sides, when the jaws or mandibles 
are closed, do not reach so far back as to pass the base 
of the eye, supposing a perpendicular line to be drawn 
from one end to the otlier. Hut in birds having a very 
wide moutl), the basal angle of the jaws passes far 
behind this imaginary line, and the mouth is said to 
open beneath the eye. The smallest mouths are seen 
in the grallatorial order, 
where we have the plovers 
(^fig. 22. a), and in the 
types by which it is re- 
presented in other circles. 
This is strictly conform 
able to that law of va- 
riation in structure before 
explained, by which this 
primary type is so distin- 
guished ; because, in fact, 
it represents the class of Acrita, in the great circle of 
the animal world, where the mouth is for the most part 
altogether wanting. It is in conformity with this law 
that the humming birds, being suctorial, have the small- 
est mouths in the insessorial order; for as they swallow 
those insects only which are very minute, a greater 
extension is not necessary. Birds, on the contrary, 
which capture their prey upon the wing, and gulp it 
down immediately, have mouths of enormous width. 
The whole tribe of the Fisnirostree, at the head of 
which are the swallows, may be instanced as possessing 
this structure, carried, perhaps, to a greater extent in 
the night-jars (Caprimulgidee, fig. 22. b) than in any 
