ORDER III. — INSESSORES. 109 



The tail of the Insessores exhibits considerable differences. 

 The number of feathers is usually twelve ; sometimes ten only, as 

 in the Strisores. 



The different groups of the order Insessores 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 jjurpose 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 Strisores, Clatnatores, and Oscines, and of their subdivisions, 

 as wUl be shown hereafter, depend very much on these peculiarities 

 of the larynx. 



The tongue of the Insessores varies to a considerable degree. 

 In the Humming-bii'ds, 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. 



