BIRDS OF NORTH AND MIDDLE AMERICA. 601 
Larus thayert Brooxs (W. 8.), Bull. Mus. Comp. Zool., lix, no. 5, Sept., 1915, 
373 (Buchanan Bay, Ellesmere Land, Arctic America, coll. Mus. Comp. 
Zool,).—ANDERSON, Summary Report Geological Survey of Canada for 1916, 
1917, 377 (Bernard Harbor; Kater Point, Arctic Sound; Cape Kellett, 
Banks Island). - 
Larus argentatus thayert Dwieut, Auk, xxxiv, 1917, 414, pl. 15 (localities; crit.). 
Larus argentatus TavERNER, Summary Report Geological Survey of Canada for 
1916. 1917, 356 (near Barkley Sound, Vancouver Island, B. C.; crit.). 
LARUS MARINUS Linneus. 
GREAT BLACK-BACKED GULL. 
Adulis in summer (sexes alike).—Head, neck, rump, upper tail- 
coverts, tail and entire under parts, including axillars and under 
wing-covertsy immaculate pure white; back, scapulars, and wings 
uniform dark sooty gray or blackish gray (between dark quaker 
drab and deep slaty brown), the tertials and secondaries broadly 
(the latter abruptly) tipped with white; outermost primary black, 
tipped with white for about 63-75 mm., the second (from outside) 
similar but more grayish basally and the white terminal area inter- 
rupted by a broad black bar on one or both webs; third, fourth, and 
fifth primaries becoming successively and more extensively, more 
grayish basally, tipped with white and with an extensive black sub- 
terminal area preceded by white, either in form of a spot or bar or 
shading gradually into the dusky gray of proximal portion; remain- 
ing (proximal) primaries dark sooty gray broadly tipped with white; 
bill deep yellow, paler at tip, the mandible red subterminally; rictus 
orange-red or vermilion (in life); iris pale yellow, whitish or silvery; 
bare orbital ring vermilion (in life); legs and feet pale flesh color. 
Adulis vn winter.—Similar to the summer plumage, but pileum and 
hindneck streaked with dusky. 
Young.—Upper parts dark slaty brown or dusky, the feathers 
broadly margined with pale dull buffy; remiges dusky or blackish, 
narrowly tipped with whitish; tail dusky, narrowly tipped with white 
and crossed on distal portion by a narrow band of grayish or brownish 
white; head, neck, and under parts dull whitish, the pileum, hind- 
neck, and sides of head and neck streaked with dusky grayish brown, 
the under parts (except throat) clouded. or irregularly spotted with 
grayish brown; bill blackish, sometimes paler basally; iris brown; 
legs and feet dull whitish (in life). 
Downy young.—General color dull grayish white, the upper parts 
marbled or irregularly spotted with dull grayish; head marked with 
irregular well-defined spots of black, as follows: Forehead with a 
narrow mesial streak; crown with two spots, one behind the other, 
with a minute spot on each side, opposite the space between the two 
larger spots; occiput with four large spots, arranged in transverse 
series, and below these three others, their position corresponding to 
