Account of Observed Forms of Larynx. 31 



position of these halt-rings is altered by the muscles fastened to their sides. In all 

 these birds the cartilaginous pessulus in the furcation of the trachea is wanting, and 

 is replaced by a tendinous strap. The memhrana tympatiiformis passes over from one 

 bronchus to the other. The bronchi consist of half-rings only. The membranous wall 

 of the organ of voice consists of two thin transparent membranes, of which the inner 

 is the mucous membrane. 



This structure is found in the genera Thamnophilus Vieill., Mi/iothera 111., Conopopliaga 

 Vieill., Chamaeza Vig., Sci/talopus Gould, Tinactor Pr. M., Furnarius Vieill., Cinclodes Graj', 

 Anabates Temm., Sptallaxis Vieill., Xenops HofTm., Dendrocolaptes Herm., and their allies. 



Thaninoplnlus. The normal strong rings of the trachea cease before the furcation, 

 and that suddenly ; there is only one strong tracheal ring at the furcation, with which 

 the bronchi are connected. In the intervening portion, the trachea is membranous and 

 transparent, and this voice-forming portion of the trachea, or, as we would call it, the 

 vocal membrane, consists generally of six, and seldom of fewer, extremely fine tracheal 

 rings of extraordinary tenuity, so that they appear as lines to the naked eye. They 

 are however ossified. The rings are soft and intercepted at the sides, and are held 

 together by an elastic longitudinal band which extends to the last ring of the trachea. 

 All this region of the trachea is very strongly flattened from before backwards. In 

 ThamnopJdlus guttaius the lowest portion of the trachea is also remarkably flattened. The 

 voice is produced by the vibrations of the half-rings, and of the thin membranous inter- 

 spaces. The voice-producing portion of the trachea is contracted by a muscle on either 

 side, which arises from the lower large rings of the trachea, and is attached to the 

 whole length of the band which holds together the half-rings, and finally to the last 

 ring, which is again perfect, but not to the bronchi. The half-rings of the bronchi are 

 in no way peculiar. 



Again, it is remarkable as regards the genus ThamnopJdlus, that in all the species 

 of it that I have examined, the ordinary depressor of the trachea, or sternotrac7iealis has 

 two heads; the stronger being attached to the firm portion of the trachea above the 

 tracheal organ of voice, the thinner, which is inferior, to the side of the lower portion 

 of the membranous division of the trachea, by the lateral band, close behind the proper 

 muscle of the larynx, at the spot where the band unites the last pair of their half-rings. 

 This head is united to the upper depressor, and both pass outwards and downwards to 

 form the sternoirachealis. See PI. II. Thamnopliihis naecim {Lanius naevius L. Gm.), Tliam- 

 nophilus guttatus Spix. {Lanius meleager Licht. Doubl. Verz. 491), Thamnopliilus cristatus 

 Pr. M., ThamnopMlus doUatus {Lanius doliatus L.) have been examined. 



