38 KEY TO THE BIRDS OF EASTERN NORTH AMERICA. 
Underparts, white; first primary, entirely black or dark brown (adult). 
Larus atricilla. Laughing Gull. 
See No. 33. 
General plumage, white; back, whites first primary entirely white, or with a blackish 
spot at tip; hind toe, with nail; bill, yellow. Pagophila alba. Ivory Gull. 
See No. 22. 
Back, pearl gray; inner web of primary, white, broadly tipped with black; a small rudi- 
Rissa tridactyle. Kittiwake Gull. 
mentary hind toe without nail. 
See No. 23. 
* Group 3. Wing, 13 to 15 inches long. 
Underparts, white; first primary entirely black, or dark brown. 
Larus atricilla. Laughing Gull. 
See No. 33. 
General plumage, white; first primary, entirely white, or with a dusky spot near tip; hind 
toe, with nail; bill, less than 1.50; wing, less than 14 3 bill, yellow. 
Pagophila alba. ory Gull, 
See No. 22. 
Underparts, white; inner web of primary, white, broadly tipped with black; a small 
Larus tridactyla. Kitttwake Gull. 
rudimentary hind toe, without nail. 
See No. 23. 
Underparts, white; terminal portion of first primary, black, with white spot near tip. 
Larus delawarensis. Ring=billed Gull. 
See No. 31. 
Primaries, pale pearl gray, becoming white at tip; bill, over 1.50; wing, over 14. 
Larus leucopterus. Iceland Gull. 
See No. 25. 
* Group 4. Wing, 15 to 17 inches long. 
Section 1. Length, over 21; bill, over 2; underparts, white. 
No black on primaries (adult). Larus glaucus. Glaucous Gull. 
See No. 24. 
Back, slaty black; primaries, black with more or less white (adult). 
Larus marinus. Great Black=backed Gull. 
See No. 27. 
Back, pale bluish, gray, or pearl gray; primaries, dull black or brownish black, more 
or less marked with white. 
American Herring Gull. 
See No. 30. 
Larus argentatus smithsonianus. 
* For directions for measurement, see page 9, 
