458 



STEUCTUEE AND CLASSIFICATION OF BIEDS 



The aftershaft is always small, and sometimes absent. 



Inilae pterylosis (according to Nitzsch) the pectoral tract 

 divides about halfway down the 

 neck ; it is broad, and gives off on 

 each side a stronger outer band. 

 The dorsal tract also divides 

 about halfway down the neck, and 

 encloses a long narrow space. 



The number of rectrices in 

 various genera is shown in the 

 annexed table. 



Pig. 216. — Mouth of 

 Bizittra lobata (aptek 



FOKBES). 

 p, pouch ; t, tongue ;fj, frsenum 

 linguse. 



The tongue is strong and 

 bordered with spines. In Mergus 

 only is it thin and pointed. 



The liver has a large gall 

 bladder. 



Biziura lobata is unique among 

 Anseres^ in having a subgular pouch (fig. 216), formed by a 

 duplicature of the frsenum linguae. 



The most noteworthy point that appears from the facts 

 given on the next page is the gradual reduction of the cseca 

 in the mergansers and smews, and the great range of variation 

 in size which they exhibit in Cygnus nigricoUis, a variation 



Nitzsch says 16. 



^ Cf . Otides for a similar structure. 



