PASSEEES. 13 



rings are reduced in thickness, leaving wide spaces filled by mem- 

 brane. The range of sounds produced by this modification is 

 much more limited than in the Tracheo-bronchial syrinx. 



In the Bronchial syrinx the voice is produced by modification 

 of the bronchi. But as this type does not occur among the 

 Passeres, its description may be deferred. 



It is not the purpose of this survey to pass in review all the 

 anatomical characters which have been used as aids to the classifi- 

 cation of this difficult Group, but rather to afford a concise summary 

 of such as are regarded to-day as of importance. 



hallux. 



...tarso-metatarsus. 



digits 



Sketcli showing the arrangement of the deep'plantar tendons in a 

 passerine bird. (From P. Z. S. 1875, p. 347.) 



After the syrinx, systematists seem to have relied most upon 

 the plantar tendons of the foot. Sometimes, indeed, too much 

 reliance seems to have been placed upon these ; that is to say, a 

 too arbitrary use has been made of the evidence they afford. 



Of these tendons two only are specially recognized in this con- 

 nexion. These are the Flexor profundus digitorum and the Flexor 

 lone/us Jiallucis. The first named arises from the greater part of 

 the hinder face of the fibula and tibia, beneath all the other 

 flexors, and at the intertarsal joint passes into a tendon, which, 

 running through a perforation in the metatarsal tubercle, divides 

 just above the distal end of the tarso-metatarsus, sending a slip 



