BIRDS OF NORTH AND MIDDLE AMERICA. 581 
Medium sized to very large Laride (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 with 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. 
r KEY TO THE SPECIES AND SUBSPECIES OF LARUS. 
a. Head, neck, rump, upper 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.) 
b. 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 subterminal spot of red. 
ce. Gray of primaries fading gradually into white terminally, without darker 
subterminal markings. 
d. Larger (wing 425-474; tail 172.5-216; culmen 49-67.3). (Circumpolar 
regions, south in winter to Long Island, Great Lakes, California, Medi- 
terranean, Black and Caspian Seas, Japan, etc.). Larus hyperboreus (p. 584). 
dd. Smaller (wing 379-394; tail 155-165; culmen 40.5-44.5). (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). 
ce. Gray of primaries succeeded by abruptly darker subterminal spaces and 
abruptly white tips. 5 
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. 
f. Smaller (wing 393-416; tail 161-166; culmen 43-47; tarsus 53-56; middle 
toe, 49.5-52.5). (Northeastern North America, south in winter to 
Connecticut and New York.).......-..-...- Larus kumlieni (p. 593). 
Of. 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 
Californias) sccsicncciaaettncaess atanurstersvaisisigicbonghots 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; Cte:) v2.00. scemexexeeseseeees cee Larus glaucescens (p. 597). 
dd. Subterminal spaces on primaries black, at least in part. 
e. Subterminal spaces on primaries partly blackish slate. (Size of D. 
kumlient, but with darker back, etc., and heavier bill.) (Ellesmere 
Land and northern Alaska.) ...............-.---- Larus thayeri (p. 600). 
