BIKDS OP NORTH AND MIDDLE AMERICA. 581 



Medium sized to very large Larida3 (wing 317-497 mm.) with the 

 well-developed hallux entirely free from inner toe; tibia with at least 

 lower third unfeathered; tarsus longer than middle toe without claw, 

 the planta tarsi not rugose or serrated; tail truncate or very slightly 

 rounded; adults with head, neck, rump, upper tail-coverts, tail and 

 entire under parts immaculate white (the head and neck streaked 

 or clouded with grayish in winter); young with rump and upper 

 tail-coverts always spotted, barred or mottled witli grayish or dusky, 

 back, scapulars, and wing-coverts streaked and mottled with grayish 

 brown and whitish or buffy, and under parts more or less washed or 

 mottled with grayish brown. 



Bill shorter than head (the exposed culmen shorter than middle 

 toe with claw), variable as to relative depth, and prominence of 

 gonydeal angle. 



KEY TO THE SPECIES AND SUBSPECIES OF LAIIUS. 



a. Head, neck, rump, iipi5er tail-coverts, tail, and entire under parts immaculate 

 white (the head and neck, at least in part, streaked or mottled with gray or 

 brown in winter). (Adults.) 

 h. Depth of bill at gonydeal angle contained less than four and a half times in 

 length of tarsus; middle toe, without claw, not less than 45 mm.; mandible 

 with a svib terminal spot of red. 

 c. Gray of primaries fading gradually into white terminally, without darker 

 sub terminal markings. 

 d. Larger (wing 425-474; tail 172.5-216; culmen 49-67.0). (('ircumpolar 

 regions, south in winter to Long Island, Great Lakes, California, Medi- 

 terranean, Black and Caspian Seas, Japan, etc.).Lanis hyperboreus (p. 584). 

 dd. Smaller (wing 379-394; tail 155-165; culmen 40.5^4.5). '(Northern Atlantic 

 and adjacent parts of Arctic Ocean, south in winter to Long Island, 

 Great Lakes, British Islands, Baltic Sea, etc.).Larus leucopterus (p. 590). 

 cc. Gray of primaries succeeded by abruptly darker subterminal spaces and 

 abruptly white tips. 

 d. Subterminal spaces on primaries gray. 

 e. Second primary (from outside) very pale gray, very broadly tipped with 

 white, the outer web with an elongated subterminal area of gray, sharply 

 defined against the paler ground color. 

 /. Smaller (wing 393-116; tail 161-166; culmen 43-1"; tarsus 53-56; m.iddle 

 toe, 49.5-52.5). (Northeastern North America, south in winter to 



Connecticut and New York.) Larus kumlieni (p. 593). 



./?'. Larger (wing, 417-468; tail, 172.5-194; culmen, 51-57; tarsus, 61-69; 

 middle toe, 54-64). (Coast of Alaska, south in winter to Lower 



California.) Larus nelsoni (p. 595). 



ee. Second primary (from outside) deep gray, either to extreme tip or else 

 with very small white tip and a small white space some distance from 

 tip, on one or both webs. (Size of L. nelsoni.) (Northern Pacific 

 Ocean and Bering Sea, south in winter to Lower California, northern 



Japan, etc.) Larus glaucescens (p. 597). 



dd. Subterminal spaces on primaries black, at least in jiart . 

 e. Subterminal spaces on primaries partly blackish slate. (Size of L. 

 kumlieni, but with darker back, etc., and heavier bill.) (Ellesraere 

 Land and northern Alaska.) Larus thayeri (p. 600). 



