PASSERES. 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 elassifi- 
cation of this difficult Group, but rather to afford a concise summary 
of such as are regarded to-day as of importance. ‘ 
tarso-metatarsus, 
Hratlltisesete- ss 2 eee 
digitst...-- 
Sketch showing the arrangement of the deep"plantar tendons in a 
passerine bird. (From P. Z. 8. 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 
tvo 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 Fleaor 
longus hallucis. ‘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 
f=) 
just above the distal end of the tarso-metatarsus, sending a slip 
