PERCHING-BIRDS. 107 
Order XXXI. PASSERIFORMES. Percuine-Birps. 
This Order, comprising the rest of the living members of the class 
Aves, includes between five and six thousand species, nearly all being 
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 egithognathous, 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 egithognathous 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. MEHSOMYODI. Soneuess Brrps. 
The Mesomyodian Passeres are further divided into two groups— 
Tracheophone and Oligomyode—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. T'racHzoPHonZ. 
Four familics are recognised, characterised by the shape of the 
sternum and by the structure of the tarsal scutes. In the Pteruptochiae 
and Conopophagide the sternum has four posterior notches, in this 
respect differing from all other Passeres; while in the Mormicartide and 
Dendrocolaptide there are only two notches. 
[Cases 
68-84. | 
