232 ZOOLOGY. 



more than six. The tail, extremely variable in shape, has 

 twelve rectrices (with certain anomalous exceptions). 

 There is but one common carotid artery, and the sternum 

 is very uniform in shape. Their high physical irritability 

 is co-ordinate with the rapidity of their respiration and 



Fig. 273.— Magpie. 



circulation; they consume the most oxygen and live the 



fastest of all birds (Coues). 



There are two groups of Passerine birds, differing in the 

 structure of the lower larynx; in the first {Clamatores) the 

 vocal organs are moi-e or less rudimentary, the species not 

 being singers, while in the second and higher division 

 {Oficines) the lower larynx is so developed that most of the 

 species excel as singers. In the singing birds the vocal 



