PERCHING-BIRDS. 107 



Order XXXI. PASSERIFORMES. PERCHING-BIRDS. 



This Order, comprising the rest of the living members of the class [Cases 

 Aves, includes between five and six thousand species, nearly all being 68 ~ 84 . 

 birds of small size. 



The feet are adapted for perching, three of the toes being directed 

 forwards and one backwards. The front toes are generally separate 

 from one another to the base. The hind toe is long, inserted low down 

 and moved by a separate tendon from that which serves the front toes, 

 so that it is capable of being powerfully opposed to them, like a thumb. 

 The palate is segithognathous, the vomer being broad and truncate and 

 the maxillo-palatine bones separate from one another (Appendix, 

 p. 145). This arrangement of the deep plantar tendons of the foot, 

 which is termed " passerine," combined with the segithognathous palate, 

 is characteristic of the Order. 



Passerine birds are divided into two sections, the Acromyodi or 

 Singing-Birds, and the Mesomyodi or Songless Passeres. This division, 

 however, is based on the anatomical structure of the syrinx or lower 

 larynx, in which the voice is produced, rather than on the actual power 

 of producing melodious notes in a certain sequence. In the Acromyodi 

 the intrinsic muscles of the syrinx are complex and consist of numerous 

 pairs fixed to the ends of the bronchial semi-rings ; while in the Meso- 

 myodi the muscles are simple, consisting in many cases of only one pair, 

 inserted into the middle of the bronchial semi-rings (Appendix, p. 148). 



All our song-birds belong to the Acromyodian group, but there are 

 numerous Passerine birds possessing the less complicated Mesomyodian 

 apparatus which can utter notes more deserving of the term song than 

 some, such as the Crows, referred to the former division. 



Section A. MESOMYODI. SONGLESS BIRDS. 



The Mesomyodian Passeres are further divided into two groups 

 Tracheophonee and Oligomyoda the former having the lower end of the 

 trachea modified to form an organ of song, while in the latter the 

 reverse obtains. 



Group I. TRACHEOPHON&. 



Four families are recognised, characterised by the shape of the 

 sternum and by the structure of the tarsal scutes. In the Pteroptochida: 

 and Conopophagidae the sternum has four posterior notches, in this 

 respect differing from all other Passeres ; while in the Formicariidcp and 

 Dendrocolaptida there are only two notches. 



