46 Field Museum of Natural History — Zoology, Vol. IX. 



Outer webs of primaries, dark brown; wing, over 17.50; depth 

 of bill at angle, over .90 (immature). Lams marinus. 



Great Black-backed Gull. 



See No. 15. 



Outer webs of primaries, dark brown; depth of bill at angle, under 

 .90; wing, under 17.50 (immature). Larus argentatus. 



American Herring Gull. 

 See No. 16. 



SECTION 3. Length, over 21; bill, under i; under parts, white. 



Back, grayish blue, more or less black on primaries. 



Larus argeyitatus. 

 American Herring Gull. 

 See No. 16. 



Back, pale pearl color; primaries, whitish or pearl color, shading 

 to white at the tips. Larus leucopterus. 



Iceland Gull. 

 See No. 14. 



SECTION 4. Length, over 21; bill, under 2; under parts, not white. 



Outer webs of primaries, dark brown; bill, over 1.90 (immature). 



Larus argentatus. 

 American Herring Gull. 



See No. 16. 



Outer webs of primaries, ash color (immature). 



Larus leucopterus- 

 Iceland Gull. 

 See No. 14. 



SECTION 5. Length, under 21. 



A band of black on the bill (adult); bill, dull yellow, tipped 

 with black (immature ) . Larus delawarensis . 



Ring-billed Gull. 



See No. 17. 



