i8 FIELD MUSEUM OF NATURAL HISTORY ZOOLOGY, VOL. IX. 



2. Hypognathous. Lower mandible longer than upper man- 

 dible. (Example: Black Skimmers, Rhynchops.} 



3. Paragnathous. Both mandibles of about equal length. 



4. Metagnathous. Mandibles crossed. (Example: Crossbill, 

 Loxia. ) 



The shape and size of the bill vary greatly, as will be seen by the 

 following illustrations representing fifteen of the principal types 

 among our birds : 



Cere. A membrane (usually hard ) , which covers the base of the 

 upper mandible. (Hawks, Parrots, Jaegers.) 



Nasal Fossa, or Nasal Groove. Groove in which the nostrils open. 



Gonys. Bend of lower mandible; usually used to include the 

 outline of the under mandible, from the angle to the tip. 



Culmen. The ridge of the upper mandible. 



