1903. 



SYRINX OF THE ACCIPITRES. 



159 



Genus PoLYBORUs. 



The syrinx of P. hrasiliensis is constituted upon the plan of 

 Falco, but there ai'e a number of differences in detail. 



There is the same extensive membi'ana tympaniformis externa 

 into which is inserted the intrinsic muscles. But the last six 

 tracheal rings are moi'e thoi-oughly fused into a box, this fusion 

 taking place in front. At the same time the last three or four of 

 these rings increase rapidly in diameter so that the lower part of 

 the trachea is veiy wide, much more so than in Falco. The 

 intrinsic muscles, moreover, are distinctly smaller in proportion in 

 Polyhorus. 



Genera Tinnunculus and Erythropus. 



The syrinx of the Common Kestrel ( T. alaudarms) appears to 

 me to need no special description. It is quite like that of Falco, 

 as is also that of Erythrops vespertinus. which gen vis perhaps hardly 

 needs to be separated from Falco. 



Genus NisAETUS. 



The syrinx of JVisaetus fasciatus (text-fig. 1 7) will serve as a 

 type of the Aquiline form of this organ as contrasted with the 

 Falconine. 



Text-fig. 17. 



Syrinx of Nisaetus fasciatus (from a drawing by tlie late Prof. Garrod) : the 

 left-hand bronchus seen from in front, the right from behind. X 2. 



The most salient difference is the absence of a specially enlarged 

 area of membrane between two of the early bronchial semirings ; 

 that is to say, the membrana tympaniformis externa is not a con- 

 spicuous feature of the syrinx of this bird, as it is of that of Falco. 

 The ring which gives rise to the pessulus in front is separated by 

 four semirings from that which receives the insertion of the 

 intrinsic muscles ; but some of these rings, three at any rate, are 



