ORDER III. — INSESSORES. 109 
The tail of the Jnsessores exhibits considerable differences, 
The number of feathers is usually twelve; sometimes ten only, as 
in the Strisores. 
The different groups of the order Jnsessores are subject to con- 
siderable variations in respect to the structure of the lower larynx 
attached to the trachea or windpipe just anterior to its division 
into the two bronchial tubes. Cuvier long since showed, that the 
true singing-birds had the larynx provided with a peculiar appa- 
ratus for the purpose of effecting a modulation of the voice, 
compesed of five pairs of muscles, of which other birds were 
destitute in greater part, or entirely. The characteristic of the 
groups Sérisores, Clamatores, and Oscines, and of their subdivisions, 
as will be shown hereafter, depend very much on these peculiarities 
of the larynx. 
The tongue of the Jnsessores varies to a considerable degree. 
In the Humming-birds, it is thread-like and bifurcated. In most 
other insessorial or perching birds, it is long or short, flat, and 
triangular, the posterior extremity bilobed, the anterior usually 
with the tip horny, serrated, or with fibres, more rarely smooth. 
These furnish important characteristics for the division into families, 
and even genera; the variations being quite considerable. 
See Introduction, and vol. IX., Pacific R.R. Reports, 128. 
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