204 NORTH AMERICAN BIRDS. 



B. Nasal bones very incompletely ossified, the nostril being a large, more or less 

 oval, opening, of oblique direction, its lower end being invariably the posterior 

 one ; without a bony tubercle, and never perfectly circular. (Accipit7-ince.) 



a. Sides of the head densely feathered close up to the eyelids. 



3. Pandion. Outer toe reversible ; claws contracted and rounded 

 on their under surface, and not graduated in size.^ Wing long, 

 third quill longest ; outer four with inner webs emarginated. Tail 

 rather short, rounded. 



4. Nauclerus. Outer toe not reversible ; claws not contracted or 

 rounded oh under side, and graduated in size. Wing long, third 

 quill longest; outer two with inner webs sinuated. Tail excessively 

 lengthened and forked, the lateral pair of feathers more than 

 twice as long as the middle pair. 



b. Sides of the head wnth a more scantily feathered orbital space, with a 

 projecting superciliary " shield " covered with a naked skin. 



* A well-developed membrane, or " web," between the outer and mid- 

 dle toes at the- base, 

 t Tarsus about equal to the middle toe. 

 § Claws short and robust ; tw^o outer quills with their inner w^ebs cut. 



5. Ictinia. Commissure irregularly toothed and notched ; front 

 of tarsus with transverse scutellae. Tail emarginated ; third quill 

 longest. 



6. Elanus. Commissure without irregularities ; front of tarsus 

 with minute roundish scales. Tail double-rounded ; second quill 

 longest. 



§§ Claws long and slender ; five outer quills with inner webs cut. 



7. Rostrhamus. End of bill bent downward, with a long pendent 

 hook ; inner edge of middle claw slightly pectinated, or serrated. 

 Tail emarginated ; third or fourth quill longest. 



ft Tarsus very much longer than the middle toe. 



IT Front of tarsus unfeathered, and, with the posterior face, covered 



with a continuous series of broad transverse scutellae. 



a. Form very long and slender, the head small, the tail and legs long and 

 claws excessively acute ; bill weak, compressed, very high through the 

 base, the culmen greatly ascending basally, and the cere much arched ; 

 commissure usually with a very prominent " festoon." 



8. Circus. Face surrounded by a " ruff" of stiffened, differently 

 formed feathers, as in the Owls. Tarsus more than twice as long 

 as the middle toe. Wing very long, hardly concave beneath; 

 third to fourth quill longest ; outer four with inner webs sinuated. 



9. Nisus. Face not surrounded by a ruff Tarsus less than 

 twice as long as the middle toe. Wing short, very concave 

 beneath, the outer quill much bowed ; third to fifth quills longest ; 

 outer five with inner webs sinuated. 



^. Form short and heavy, the head larger, the tail shorter, the legs more 

 robust. Bill strongei-, less compressed, lower through the base, the 

 upper outline less ascending basally, and the cere less arched. Com- 

 missure variable. 



^ By this is meant that they are all of equal length and thickness, and not progressively 

 smaller from the posterior one to the oxiter, as in all Falconidce with the sole exception of Pandion, 

 though there is a very near approach to this feature in one or two of the species of Ilalicetus. 



