4 
BEAUTIFUL BIEDS. 
the hook of the upper mandible, and thus allows the 
lower one to be opened, so that the fragments of the 
shell may be rejected. 
Por the accomplishment of this action in the low'er 
mandible, it is supplied with a set of very powerful 
muscles, and it has a sliding or grinding motion, 
which differs with the degree of exertion. It is 
pushed a little forwards at the same time that it is 
raised by the muscles. The upper mandible has 
much less motion than the lower, but still it has more 
than in most birds. 
To prevent any concussion which might be com- 
municated to the brain, the upper mandible is articu- 
lated with the bones of the cranium, and not united. 
In consequence of this, the upper mandible is sus- 
ceptible of some motion on its base ; and this motion 
not only prevents the concussion from being propa- 
gated to the brain, but enables this organ to exert a 
much more powerful effect than it would have, if the 
upper mandible were so firmly united at its base as 
to be incapable of motion. The bill thus not only 
becomes a powerful crunching instrument, but is ren- 
dered effective likewise in taking hold with a firm 
gripe of small boughs, to assist the bird in its pro- 
gress amongst them. To render the upper mandible 
more effective as a rest against the pressure of the 
lower one, the under side is slightly hollowed towards 
the hook, and roughened like a millstone, by means 
of angular furrows, with their apices directed towards 
the tip. 
There are several variations in the form of the bill 
in this numerous family. The Paroquets have the 
