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



Under parts, white; inner web of first primary, white; the tip, 

 black; bill, black; shaft of feather, white. Larus Philadelphia. 



Bonaparte's Gull. 



See No. 20. 



Under parts, white; inner web of first primary, about half white; 

 shaft of feather, dark brown on upper surface. In the adult the bill is 

 black, tipped with yellow. Xenta sabinii. 



Sabine's Gull. 

 See No. 21. 



*GROUP 2. Wing, n to 13 inches long. 



Shafts of primaries, black or dark brown; under parts, not pure 

 white (immature ) . Larus atricilla. 



Laughing Gull. 

 See No. 18. 



Under parts, pure white; inner web of first primary, white, with 

 black spot near the end, the tip, white; shaft of feathers, white. 

 Adult birds have bill dark red; in immature birds it is brownish. 



Larus franklinii. 

 Franklin's Gull. 

 See No. 19. 



Under parts, white; first primary, entirely black or dark brown 

 (adult ) . Larus atricilla. 



Laughing Gull. 

 See No. 18. 



Back, pearl gray; inner web of primary, white, broadly tipped 

 with black; a small rudimentary hind toe, without nail. 



Rissa tridactyla. 

 Kittiwake Gull. 

 See No. 12. 



*GROUP 3. Wing, 13 to 15 inches long. 



Under parts, white; first primary entirely black, or dark brown. 



Larus atricilla. 

 Laughing Gull. 

 See No. 18. 



*For directions for measurement, see page 21. 



