172 STRUCTURE AND CLASSIFICATION OF BIRDS 



PASSERES 



Definition. Oil gland nude. Skull segithognathous. Atlas perforated 

 by odontoid process. One carotid, left. Caeca present, small.' 

 Muscle formula, AXY.- No biceps slip or expansor secunda- 

 riorum. 



This is an enormous group of birds, numbering over 

 6,000 species, which are spread over the entire globe. As a 

 rule they are of small or moderate size ; but some large 

 species, such as the raven and other Corvidse, are included 

 in the assemblage. In spite of the numerous species and of 

 a certain amount of differentiation in external form, the group 

 is structurally a uniform one, the difference being in cha- 

 racters which have not as a rule been regarded as of primary 

 importance. 



In all Passeres the foot has this structure : the first toe is 

 directed backwards, and none of the other toes are ever 

 changed in position. Cliolornis is exceptional in that the 

 fourth toe is abortive. 



GAEEOD noted, some years since, another peculiar character 

 of the group, which may possibly be universal, with the ex- 

 ception of Menura and Atrichia. This concerns the arrange- 

 ment of the tendon of the patagialis brevis ; and the passerine 

 disposition may be understood from a comparison of the two 

 descriptions. In the passerine the tendon of the muscle 

 does not end upon the tendon of the extensor, as it does 

 in the picarian bird, but, though attached to it firmly, 

 retains its independence and runs back to be attached near 

 it to the extensor condyle of the radius. This difference, 

 though small, appears to be constant to the Passeres. Another 

 character dealt with on p. 41 of the present work may be 

 also an exclusively passerine character. In birds belonging 

 to the present family the oblique septa, instead of having a 

 separate attachment to the sternum, are either not attached 



1 In a specimen of Gracula intermedia the casca were as long as half an 

 inch, an exceptional length. 

 ' Very rarely AX . 



